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Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary information encoded in genes, which can be transmitted to future generations. Another major theme is evolution, which explains the unity and diversity of life. Finally, all organisms require energy to move, grow, and reproduce, as well as to regulate their own internal environment.

Biologists are able to study life at multiple levels of organization. From the molecular biology of a cell to the anatomy and physiology of plants and animals, and evolution of populations. Hence, there are multiple subdisciplines within biology, each defined by the nature of their research questions and the tools that they use. Like other scientists, biologists use the scientific method to make observations, pose questions, generate hypotheses, perform experiments, and form conclusions about the world around them.

Life on Earth, which emerged more than 3.7 billion years ago, is immensely diverse. Biologists have sought to study and classify the various forms of life, from prokaryotic organisms such as archaea and bacteria to eukaryotic organisms such as protists, fungi, plants, and animals. These various organisms contribute to the biodiversity of an 

Biology derives from the Ancient Greek words of βίος romanized bíos meaning 'life' and -λογία; romanized -logía meaning 'branch of study' or 'to speak'. Those combined make the Greek word βιολογία romanized biología meaning 'biology'. Despite this, the term βιολογία as a whole didn't exist in Ancient Greek. The first to borrow it was the English and French (biologie). Historically there was another term for biology in English, lifelore; it is rarely used today.

The Latin-language form of the term first appeared in 1736 when Swedish scientist Carl Linnaeus (Carl von Linné) used biologi in his Bibliotheca Botanica. It was used again in 1766 in a work entitled Philosophiae naturalis sive physicae: tomus III, continens geologian, biologian, phytologian generalis, by Michael Christoph Hanov, a disciple of Christian Wolff. The first German use, Biologie, was in a 1771 translation of Linnaeus' work. In 1797, Theodor Georg August Roose used the term in the preface of a book, Grundzüge der Lehre van der Lebenskraft. Karl Friedrich Burdach used the term in 1800 in a more restricted sense of the study of human beings from a morphological, physiological and psychological perspective (Propädeutik zum Studien der gesammten Heilkunst). The term came into its modern usage with the six-volume treatise Biologie, oder Philosophie der lebenden Natur (1802–22) by Gottfried Reinhold Treviranus, who announced:

The objects of our research will be the different forms and manifestations of life, the conditions and laws under which these phenomena occur, and the causes through which they have been affected. The science that concerns itself with these objects we will indicate by the name biology [Biologie] or the doctrine of life [Lebenslehre].

The earliest of roots of science, which included medicine, can be traced to ancient Egypt and Mesopotamia in around 3000 to 1200 BCE. Their contributions later entered and shaped Greek natural philosophy of classical antiquity. Ancient Greek philosophers such as Aristotle (384–322 BCE) contributed extensively to the development of biological knowledge. His works such as History of Animals were especially important because they revealed his naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany that survived as the most important contribution of antiquity to the plant sciences, even into the Middle Ages.

Scholars of the medieval Islamic world who wrote on biology included al-Jahiz (781–869), Al-Dīnawarī (828–896), who wrote on botany, and Rhazes (865–925) who wrote on anatomy and physiology. Medicine was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.

Biology began to quickly develop and grow with Anton van Leeuwenhoek's dramatic improvement of the microscope. It was then that scholars discovered spermatozoa, bacteria, infusoria and the diversity of microscopic life. Investigations by Jan Swammerdam led to new interest in entomology and helped to develop the basic techniques of microscopic dissection and staining.

Advances in microscopy also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the cell. Then, in 1838, Schleiden and Schwann began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of life, although they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work of Robert Remak and Rudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known as cell theory.

Meanwhile, taxonomy and classification became the focus of natural historians. Carl Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species. Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Although he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought; his work influenced the evolutionary theories of both Lamarck and Darwin.

Serious evolutionary thinking originated with the works of Jean-Baptiste Lamarck, who was the first to present a coherent theory of evolution. He posited that evolution was the result of environmental stress on properties of animals, meaning that the more frequently and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could then be passed on to the animal's offspring, who would further develop and perfect them. However, it was the British naturalist Charles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell, Malthus's writings on population growth, and his own morphological expertise and extensive natural observations, who forged a more successful evolutionary theory based on natural selection; similar reasoning and evidence led Alfred Russel Wallace to independently reach the same conclusions. Darwin's theory of evolution by natural selection quickly spread through the scientific community and soon became a central axiom of the rapidly developing science of biology.

The basis for modern genetics began with the work of Gregor Mendel, who presented his paper, "Versuche über Pflanzenhybriden" ("Experiments on Plant Hybridization"), in 1865, which outlined the principles of biological inheritance, serving as the basis for modern genetics. However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the modern synthesis reconciled Darwinian evolution with classical genetics. In the 1940s and early 1950s, a series of experiments by Alfred Hershey and Martha Chase pointed to DNA as the component of chromosomes that held the trait-carrying units that had become known as genes. A focus on new kinds of model organisms such as viruses and bacteria, along with the discovery of the double-helical structure of DNA by James Watson and Francis Crick in 1953, marked the transition to the era of molecular genetics. From the 1950s to the present times, biology has been vastly extended in the molecular domain. The genetic code was cracked by Har Gobind Khorana, Robert W. Holley and Marshall Warren Nirenberg after DNA was understood to contain codons. Finally, the Human Genome Project was launched in 1990 with the goal of mapping the general human genome. This project was essentially completed in 2003, with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.

All organisms are made up of matter and all matter is made up of elements. Oxygen, carbon, hydrogen, and nitrogen are the four elements that account for 96% of all organisms, with calcium, phosphorus, sulfur, sodium, chlorine, and magnesium accounting for the remaining 3.7%. Different elements can combine to form compounds such as water, which is fundamental to life. Life on Earth began from water and remained there for about three billions years prior to migrating onto land. Matter can exist in different states as a solid, liquid, or gas.

The smallest unit of an element is an atom, which is composed of an atomic nucleus and one or more electrons moving around the nucleus, as described by the Bohr model. The nucleus is made of one or more protons and a number of neutrons. Protons have a positive electric charge, neutrons are electrically neutral, and electrons have a negative electric charge. Atoms with equal numbers of protons and electrons are electrically neutral. The atom of each specific element contains a unique number of protons, which is known as its atomic number, and the sum of its protons and neutrons is an atom's mass number. The masses of individual protons, neutrons, and electrons can be measured in grams or Daltons (Da), with the mass of each proton or neutron rounded to 1 Da. Although all atoms of a specific element have the same number of protons, they may differ in the number of neutrons, thereby existing as isotopes. Carbon, for example, can exist as a stable isotope (carbon-12 or carbon-13) or as a radioactive isotope (carbon-14), the latter of which can be used in radiometric dating (specifically radiocarbon dating) to determine the age of organic materials.

Individual atoms can be held together by chemical bonds to form molecules and ionic compounds. Common types of chemical bonds include ionic bonds, covalent bonds, and hydrogen bonds. Ionic bonding involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compounds. Ions are atoms (or groups of atoms) with an electrostatic charge. Atoms that gain electrons make negatively charged ions (called anions) whereas those that lose electrons make positively charged ions (called cations).

Unlike ionic bonds, a covalent bond involves the sharing of electron pairs between atoms. These electron pairs and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding.

A hydrogen bond is primarily an electrostatic force of attraction between a hydrogen atom which is covalently bound to a more electronegative atom or group such as oxygen. A ubiquitous example of a hydrogen bond is found between water molecules. In a discrete water molecule, there are two hydrogen atoms and one oxygen atom. Two molecules of water can form a hydrogen bond between them. When more molecules are present, as is the case with liquid water, more bonds are possible because the oxygen of one water molecule has two lone pairs of electrons, each of which can form a hydrogen bond with a hydrogen on another water molecule.

Life arose from the Earth's first ocean, which was formed approximately 3.8 billion years ago. Since then, water continues to be the most abundant molecule in every organism. Water is important to life because it is an effective solvent, capable of dissolving solutes such as sodium and chloride ions or other small molecules to form an aqueous solution. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in chemical reactions that sustain life.

In terms of its molecular structure, water is a small polar molecule with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H2O). Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge. This polar property of water allows it to attract other water molecules via hydrogen bonds, which makes water cohesive. Surface tension results from the cohesive force due to the attraction between molecules at the surface of the liquid. Water is also adhesive as it is able to adhere to the surface of any polar or charged non-water molecules.

Water is denser as a liquid than it is as a solid (or ice). This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby insulating the liquid below from the cold air above. The lower density of ice compared to liquid water is due to the lower number of water molecules that form the crystal lattice structure of ice, which leaves a large amount of space between water molecules. In contrast, there is no crystal lattice structure in liquid water, which allows more water molecules to occupy the same amount of volume.

Water also has the capacity to absorb energy, giving it a higher specific heat capacity than other solvents such as ethanol. Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into gas (or water vapor).

As a molecule, water is not completely stable as each water molecule continuously dissociates into hydrogen and hydroxyl ions before reforming into a water molecule again. In pure water, the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a pH that is neutral. If hydrogen ions were to exceed hydroxyl ions, then the pH of the solution would be acidic. Conversely, a solution's pH would turn basic if hydroxyl ions were to exceed hydrogen ions.