A colloquialism is a word, phrase, or paralanguage that is employed in conversational or informal language but not in formal speech or formal writing. Dictionaries often display colloquial words and phrases with the abbreviation colloq. as an identifier. Colloquialisms are sometimes referred to collectively as "colloquial language".

Colloquialisms include words (such as y'all, gonna, and wanna), phrases (such as old as the hills, raining cats and dogs, and dead as a doornail) and aphorisms (such as There's more than one way to skin a cat).

Generally, colloquialisms are specific to a geographical region; although, they are also spread through "everyday" conversation and, increasingly, through informal online interactions. An example of the regional specificity of colloquialisms is the term used when referring to "soft drinks". In the Upper Midwestern United States and Canada, soft drinks are called "pop", whilst in other areas, notably the Northeastern and far Western United States, they are referred to as "soda". In some areas of Scotland, the term "ginger" is used.