What is the sum of words that a person knows and can use?

There isn't a single, universally accepted term for the sum of words a person knows and can use. However, here are some terms that come close:

* Vocabulary: This is the most common term used to describe the total number of words a person knows. It encompasses both active and passive vocabulary, meaning words they can use in speaking and writing, as well as words they understand when reading or listening.

* Lexicon: This term refers to the entire vocabulary of a language, but it can also be used to describe a person's individual vocabulary.

* Word knowledge: This term is broader than just the number of words, encompassing a person's understanding of words, their meanings, and how they are used in different contexts.

While there's no single perfect term, "vocabulary" is the most widely used and understood term to describe the sum of words a person knows and can use.