Idiolect

The speech or ‘dialect’ of a single individual. Hence idiolectal variation is variation within a language between one speaker and another.
The Concise Oxford Dictionary of Linguistics P. H. Matthews

An idiolect, in this sense, belongs to a single individual, in the sense that one’s idiolect reflects one’s own linguistic capabilities and, in that sense, is fully determined by facts about oneself.
The specific way that a single person speaks. This includes a person's
vocabulary, grammar, pronunciation, and anything else that affects how words come
out of their mouth. It comes from the Greek idio- meaning “one’s own” and -lect
which technically comes from Greek but has become a common linguistic ending
because of the word “dialect.” The prefix idio- isn’t super common, but you can also
see it in the word “idiosyncrasy.”
An idiolect is a language, the linguistic properties can be exhaustively
specified in terms of the intrinsic properties of some single individual, the person
whose idiolect it is. The force of “intrinsic” is to exclude essential reference to
features of the person’s wider environment, particularly their linguistic community.

Sources:

Idiolect linguistics. Online Encyclopedia Britannica. Retrieved from: https://www.britannica.com/topic/dialect#ref1046707

Idiolect. Babbel Magazine. Retrieved from: https://www.babbel.com/en/magazine/what-is-an-idiolect

Idiolect. Stanford Encyclopedia of Philosophy. Retrieved from: https://plato.stanford.edu/entries/idiolects/