Linguistic Terminology

One of the big issues in classification, mostly a terminology question, is where to draw the line between two ways of speaking and writing being dialects v. different languages, in part, because it often has political connotations.

"language" has two meanings: a generalized, abstract sense that comprises all human speech and writing, and the officially recognized speech and writing of a nation / country / gens — a politically united group of people.

A topolect is the speech / writing of the people living in a certain place or area. It is geographically determined.

A dialect is a distinctive form / style / pronunciation / accent shared by two or more people. To qualify as the speaker of a particular dialect, one must possess a pattern of speech, a lect, that is intelligible to others who speak the same dialect. As we say in Mandarin, it's a question of whether what you speak is jiǎng dé tōng 講得通 ("mutually intelligible") or jiǎng bùtōng 講不通 ("mutually unintelligible"). If what two people are speaking is jiǎng bùtōng 講不通 ("mutually unintelligible"), then they're not speaking the same dialect.

Naturally, the dividing line between one dialect and another is not sharp. There is a blending, a gradation, a blurring between them. The same if true of languages on a larger scale. For example, I can understand close to a 100% of the speech of natives of Stark County, Ohio, but maybe only 75-80% of rapid speech from Vinton County in the south.

An idiolect is spoken by only one person. . . .

"Dialect" means so many radically different things, but also so many things that somewhat resemble each other, but are not really the same, that it is essentially useless for scientific purposes.

From Language Log.