What term, ideally used by linguists, refers to the entire collection of words that a person knows? For example, if someone opened up a dictionary, and created a list of all of the words they know, what term would refer to that list?
You mean one's (personal) vocabulary?
all of the words known and used by a person
I would use the word lexicon.
noun the vocabulary of a person, language, or branch of knowledge.
Quoted from Oxford Dictionaries Online.
There is also a colloquial word for this: wordrobe. It is like a wardrobe of words.
a person's vocabulary
She reads dictionaries to build up her wordrobe.
Also word-hoard ( from TFD) :
(n.) the sum of words one uses or understands; a vocabulary.
a person's vocabulary.
Vocabulary, as already posted by @oerkelens, is the stock of words used by or known to a particular person.
- "He has a somewhat limited vocabulary."
- "Reading will certainly help you expand your vocabulary."
- "My wife has a wide vocabulary."
idiolect: the variety or form of a language used by an individual http://dictionary.reference.com/browse/idiolect?s=t