The term 'register' in the game of English vocabulary?

Register, in linguistics, is a speech variety used by a particular group of people, usually sharing the same occupation or the same interests.

Speaking according to the social context and standing of the user, people within the same register speak at a similar level of in/formality and use similar choice of vocavulary. Their pronunciation and syntax are similar.

You can see the considerable difference in the ways a group of lawyers and a group of doctors use language to communicate among themselves, that is, among their respective groups. The register of a particular group of teenage boys (of a partcular area at a particular point in the language's history) interested in biking stunts would be strikingly different again from the register of these two groups.