A generalized term for any form of language spoken by a smaller group of its speakers. It originates from the word 'dialect', a regional variety, from the Greek root leg- 'to speak'.

From this developed sociolect, the language of one class, and idiolect, the language of a single person, since we each speak slightly differently from our neighbours. Less happily formed is genderlect, a language variety characteristic of one sex.

Socially, we vary our speech depending on the situation, from more formal or careful to more colloquial or casual: the formal may also be perceived as more 'correct' by some people. This range of registers, as they are called, is expressed by the terms acrolect (high speech), basilect (low speech), and mesolect in the middle. These terms are also particularly used with reference to creoles, where there is often a continuum of features, with the basilect most creole-like and the acrolect closest to the parent language.

I learn from liveforever's write-up there that there is also such a thing as a cryptolect, a hidden lect such as thieves' cant.

The adjective of 'lect' is 'lectal', though I can't say I've ever seen it used.