Definition - A variety of a language that is used on certain occasions or in certain situations.

Example -
An English speaking male in his 20s might refrain from using the F-word and other swear words when eating Sunday dinner with his relatives, whereas he might not hesitate to use them when having beers in a pub with his buddies.

Etymology -
The term was coined by linguist Thomas Bertram Reid in 1956.

Oxford English Dictionary -
Its first citation is from 1956:
"He will on different occasions speak (or write) differently according to what may roughly be described as different social situations: he will use a number of distinct ‘registers’."
(T. B. W. Reid in Archivum Linguisticum VIII. 32)

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