add glossary

docs/reference/glossary.md

@@ -1,7 +1,19 @@
 # Glossary of Terms
 
 ## Cover
+A cover sheet of a spreadsheet. Contains information about the dataset(s) in the Excel workbook,
+such as publication dates and methodology notes. Handled by the [`Cover`](../api/classes/cover.md) class.
 
-## gptables
+## Element
+A [part of a table](../getting_started/elements.md), such as a title or subtitle. Relevant when defining values of elements in a `GPTable`.
+
+## GPTable
+A [`Good Practice Table object`](../api/classes/gptable.md), which has data and elements such as title, subtitles, and column units
+associated with it.
+
+## Placeholder
+A [note](../getting_started/tutorial.md#adding-notes) reference denoted by double dollar notation, e.g. `$$placeholder_text$$`. These are
+replaced by numbers when the workbook is written to file.
 
 ## Theme
+A [.yaml file](../how_to/custom_theme.md) describing pre-set formatting options.

mkdocs.yml

@@ -29,7 +29,7 @@ nav:
   - Reference:
     - Accessibility checklist: reference/checklist.md
     - Examples: reference/examples.md
-    #- Glossary: reference/glossary.md
+    - Glossary: reference/glossary.md
   - About GPTables:
     - Contributing guidance: info/contributing.md
     - Changelog: info/changelog.md