From IFWiki

In interactive fiction, any group of text recognized by the parser as a command. Most commands recognized by a game begin with words that are ordinary verbs in the grammatical sense (e.g. "take lantern", "fill bottle", "quit"), but other inputs contain only implied verbs that are supplied internally by the game's grammar (e.g. "inventory", "north", or "again"). The designer who supplies new locations and new objects must usually supply new verbs and specialized grammar, expanding the game's world, and the narratives encountered by the player who traverses the game.

-- DGJ - 10 Sep 2002