Colloquialism, n.
A colloquial expression, not employed in formal discourse or writing.
Shades of meaning
Fernald has no entry of its own for colloquialism; he lists it under slang.
Where he writes about it:
A colloquialism is an expression not coarse or low, and perhaps not incorrect, but below the literary grade; educated persons are apt to allow themselves some colloquialisms in familiar conversation, which they would avoid in writing or public speaking. — from slang