An idiolect is the distinctive speech of an individual, a linguistic pattern regarded as unique among speakers of a person's language or dialect. But it is even more granular, more narrow than just all the speakers of a particular dialect.
"Analyzing English Grammar" notes:
Because each of us belongs to different social groups, we each speak a language variety made up of a combination of features slightly different from those characteristics of any other speaker of the language. The language variety unique to a single speaker of a language is called an idiolect. Your idiolect includes the vocabulary appropriate to your various interests and activities, pronunciations reflective of the region in which you live or have lived, and variable styles of speaking that shift subtly depending on whom you are addressing.
(Thomas P. Klammer, Muriel R. Schulz, and Angela Della Volpe. Longman, 2007)
The term idiolect-made up of the Greek idio (personal, private) + (dia)lect-was coined by linguist Bernard Bloch. In linguistics, idiolects fall under the study of linguistic variation, such as dialects and accents.
In an article for Slate, author Gretchen McCulloch explained further how deep a person's idiolect goes and how people come up with their own take on their language.
A person's idiolect is not just vocabulary; it's everything from how we pronounce certain words to how we put them together to what we imagine they mean. Ever have a disagreement with someone over whether an ambiguously-shaded object was actually blue or green? Congratulations, you've witnessed differences in idiolect…
Your sense of English as a whole is really an abstract combination of all of the idiolects that you've experienced over the course of your life, especially at a young and formative age. The conversations you've had, the books you've read, the television you've watched: all of these give you a sense of what exists out there as possible variants on the English language. The elements that you hear more commonly, or the features that you prefer for whatever reason, are the ones you latch onto as prototypical.
("Why Do You Think You're Right About Language? You're Not." May 30, 2014)
To illustrate just how individual an idiolect can be, take this dialogue from Tom, played by Aziz Ansari, in "Parks and Recreation," where he explains his own personal "slanguage":
Zerts are what I call desserts. Tray-trays are entrees. I call sandwiches sammies, sandoozles, or Adam Sandlers. Air conditioners are cool blasterz, with a z. I don't know where that came from. I call cakes big ol' cookies. I call noodles long-a** rice. Fried chicken is fri-fri chicky-chick. Chicken parm is chicky chicky parm parm. Chicken cacciatore? Chicky catch. I call eggs pre-birds or future birds. Root beer is super water. Tortillas are bean blankies. And I call forks… food rakes. (2011)
Difference Between Idiolect and Dialect
A person's idiolect also includes the levels of diction or language that he or she uses in different social situations.
Zdeněk Salzmann noted in "Language, Culture, and Society":
Almost all speakers make use of several idiolects, depending on the circumstances of communication. For example, when family members talk to each other, their speech habits typically differ from those any one of them would use in, say, an interview with a prospective employer. The concept of idiolect refers to a very specific phenomenon-the speech variety, or linguistic system, used by a particular individual. All those idiolects that have enough in common to appear at least superficially alike belong to a dialect. The term dialect, then, is an abstraction.
Being an abstraction, then, makes it tough to quantify and define clearly, as Patrick R. Bennett noted in "Comparative Semitic Linguistics." At various times:
… linguists have tried to set criteria, to say that two idiolects are members of the same dialect if they have this much in common or are to this degree mutually intelligible, but they pertain to the same language if there are greater differences. But all of the cutoff points are arbitrary. (1998).
And William Labov laments, in "Sociolinguistic Patterns":
It must be noted that the very existence of the term 'idiolect' as a proper object of linguistic description represents a defeat of the Saussurian notion of langue as an object of uniform social understanding.
(University of Pennsylvania Press, 1972)