Vernacular (adj.) (from the Latin for “native slave", or "slave born in the master’s house”) refers to language that is native to, or originating in, the place where it is spoken. This is against “literary” or “learned” language. In short, it is the language of the common folk. An example would be when the Catholic church ceased using Latin in its services and began holding them “in the vernacular” (the language of the country or region).
Vernacular can also mean slang or even the vocabulary of a particular profession or trade.
Reference O. E. bloody D.