The Definitions tool allows for a definition to be applied to a few words or a whole sentence. As with abbreviations, hovering over the text will show the definition.
The Abbreviation Tool allows for a short string of capital letters that can be applied to abbreviate a word. They can be applied simply by typing in two or more letters in capitals, at which point the system will ask if you are wanting to create/insert an abbreviation.
Note: If you see the background color turn red, it means that the abbreviation or the definition is outdated. You can easily update outdated abbreviations and definitions by double-clicking on them!