What is the term for the mental collection of all the words we know?

The term that refers to the mental collection of all the words we know is "lexicon." In cognitive psychology and linguistics, a lexicon encompasses not just individual words but also the information related to those words, such as their meanings, uses, and relationships with other words. It acts as an internal database of the language skills a person possesses, including their knowledge about syntax, morphology, and semantics.

While vocabulary often refers to the set of words understood or used by an individual, it does not capture the broader cognitive aspects that the term lexicon includes, such as the mental representations and contextual understandings of those words. A dictionary, on the other hand, is a physical or digital resource containing definitions and descriptions of words but does not pertain to an individual's mental representation of language. Similarly, a thesaurus is a tool that provides synonyms and antonyms for words, serving a different purpose in language use.

In summary, the lexicon is the most comprehensive term for the mental repository of words and their meanings that an individual holds, making it the correct choice in this context.