Definition - The middle register of a spoken language. Lower than the high prestige acrolect, but higher that the informal basilect. It is usually the most widely spoken version of a language.

Etymology -
The term was coined by Derek Bickerton by — no doubt — combining the Greek mesos, middle, with 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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