What term describes informal words or expressions not usually acceptable in formal writing?

The term that describes informal words or expressions that are not typically acceptable in formal writing is colloquialism. Colloquialisms are phrases or words that are used in everyday conversation but may not be appropriate in formal contexts. They often reflect the speech patterns of a particular group or region, making them relatable and familiar in casual settings.

While jargon refers to specialized terminology used within specific fields or professions, it maintains a more formal tone within its context. Dialect pertains to the forms of language unique to a particular group or region, incorporating specific grammatical structures and vocabulary, which may not always fall into the category of informality. Slang also represents informal language, often characterized by creative or trendy terms, but it tends to be very specific to particular social groups and can change quickly over time. Thus, colloquialism best encapsulates the idea of informal language that might not be deemed suitable for formal writing scenarios.