Vernacular
Etymology
From Latin vernaculus, "domestic, native, homegrown," from verna, a slave born in the master’s house rather than bought abroad. The word’s core is thus "belonging to home." That sense survived intact: the vernacular is the home-born language of a place, once contrasted with imported, high-status Latin.
noun · /vur-NAK-yuh-lur/
The everyday language ordinarily spoken by the people of a place, as opposed to a formal or official tongue.
What it names
The living, homegrown speech of a community — what people actually say at the kitchen table, distinct from the prestige language of law, church, or school.
Example
Writing his treatise in the vernacular instead of Latin, he reached readers no scholar had bothered to address.
Related idea
Vernacular is the ordinary language of a whole community; colloquial describes the informal register within a language. Vernacular contrasts with a foreign or formal tongue; colloquial contrasts with the buttoned-up way you’d write it down.