Definition - In a post-creole community, it is the dialect that most closely resembles the creole. It is also the lowest register of a spoken language.

Etymology -
The term was coined by Derek Bickerton by — I would guess — joining the combining form basi, base, with the word dialect.

Oxford English Dictionary -
Its first citation is from 1977:
"Speakers in a post-creole community are triply pressured:
to avoid the basilect, to acquire the acrolect, and to vary the mesolect."
(Language LIII. 330 )

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