"When a group of speakers of a particular language differs noticeably in its speech from another group, we say that the groups are speaking different dialects... Linguistically speaking, ... a dialect is any variety of a language spoken by a group of people that is characterized by systematic differences from other varieties of the same language in terms of structural or lexical features. In this sense, every person speaks a dialect of her native language."
Vedrana Mihalicek and Christina Wilson (Eds.) 2011. Language Files: Materials for an Introduction to Language and Linguistics. Columbus: The Ohio State University Press,409.