The mode that
vi is in when you first run it. The
editor exists in two distinct modes, the other of which is
insert mode.
In command mode the user can move the cursor, search and replace, read and write files, define macros, pipe parts of the buffer through shell commands... pretty much everything except enter the actual text of the file. The latter is instead done in insert mode.
New users are often confused and annoyed by this duality; however the enlightened realise that the mode concept is key to the editor's extreme power and efficiency. For example, the speed, comfort and RSI-avoidance afforded by being able to move the cursor with the hjkl keys would be impossible were it not for modedness.