Colloquial

Etymology

From Latin colloquium, "a conversation," from com- ("together") + loqui ("to speak"). The literal idea is simply "speaking together." English fashioned the adjective in the eighteenth century to mark the words and turns of phrase native to conversation, as distinct from the elevated diction of formal prose.

adjective · /kuh-LOH-kwee-ul/
Belonging to ordinary, informal conversation rather than to formal or literary writing.
What it names
The relaxed register of how people actually talk — a neutral label, not an insult, for language that would look out of place in a contract.
Example
The dictionary flags "gonna" as colloquial, not wrong — fine in speech, odd in a cover letter.
Related idea
Colloquial language is casual but widely shared; slang is newer, more in-group, and quicker to date. "Kids" is colloquial; "rugrats" leans toward slang.