Word completion helps you enter words faster by typing just a few letters and selecting a complete word from the candidate list, such as when entering reserved words in programming languages.
In the CotEditor app 
on your Mac, open a document.
Type the first few letters of a word.
Choose Edit > Complete, or press Option-Esc or F5.
Select the word you want to enter from the list.
To close the list without completing a word, press the Esc key.
In the Completion section of the Mode settings pane, you can change the types of completion candidates that appear in the list. For example, the list can include only words from the document or additional words provided by the selected syntax.