obsolescence (n.)

In HISTORICAL LINGUISTICS, a term used to describe the gradual loss of a LEXICAL item because changes in the language or in the external world eliminate the opportunity or motivation for its use. Examples of obsolescent words in English would be the terms referring to vehicles from a previous era, such as landau or hansom. These words have not gone completely out of use, as they will be heard from time to time at vintage rallies and in other special contexts, but most people would not use them. When a word does go totally out of general use, it is said to be obsolete – as with comptable and comradery.

In SOCIOLINGUISTICS, a term used to describe the gradual loss of a LANGUAGE, which takes place when its transmission between generations ceases, and the number of its native speakers diminishes.