docs(multi-dim): basic README with lingo

site/multiDim/README.md

@@ -0,0 +1,37 @@
+## Lingo
+
+### MDD
+
+This directory contains code regarding multi-dimensional data pages, or MDDs for short.
+
+### Dimension
+
+A _dimension_ is one configurable aspect of the MDD, containing a multitude of _choices_.
+A MDD usually has 2-5 dimensions.
+Every dimension has a slug and a name, and the slug should not be changed once published.
+
+Example: Dimension `Age group` with choices `All`, `0-14 years`, `15-64 years`, `65+ years`.
+
+### Choice
+
+A _choice_ is a particular option inside a dimension.
+A choice has a slug and a name, and the slug should not be changed once published.
+
+### Choice groups
+
+Choices may be grouped into choice groups, if that's useful.
+This is purely a UX consideration and doesn't affect the MDD in any other way.
+
+For example, for a `Energy source` dimension, you may group the choices `Coal`, `Oil` and `Gas` under `Fossil fuels`.
+
+### View
+
+A view is a MDD configuration specified by the choices made in all dimensions.
+It can also be understood as a grapher & datapage configuration object, which then decides what to render on the page below the dimension dropdowns.
+There is at most one view for a given set of dimension-choice pairings.
+
+### Indicators
+
+An indicator or variable is an OWID indicator used to render a chart.
+A view may specify one or more y indicators, and up to one x, size, and color indicator.
+The first y indicator is considered the "main" indicator for a MDD, and its metadata is used to render the datapage information outside the charting area.