1. Manner of speaking, language, speech; especially a manner of speech peculiar to, or characteristic of, a particular person or class; phraseology, idiom.
2. One of the subordinate forms or varieties of a language arising from local peculiarities of vocabulary, pronunciation, and idiom.
3. Any form of speech considered a deviation from real or imaginary standard speech.

In relation to modern languages, usually specifically, a variety of speech differing from the standard or literary language; a provincial method of speech; such as, in speakers of dialect.

Also in a wider sense applied to a particular language in its relation to the family of languages to which it belongs.