Vocabulary refers to the words a creature knows. Usually, only proper words are included and not bibble. For example if a creature knows the words "eat", "come", "run", "hand" and "grendel" it is said to have a vocabulary of five words.
Vocabulary can also refer to a predetermined list of words in the game. Instant vocabulary means to teach a creature all these words at once. Agents such as the Holistic Learning Machine can do this.