A colloquialism is an expression not used in formal speech, writing or paralinguistics. Colloquialisms denote a manner of speaking or writing that is characteristic of familiar "common" conversation; informal colloquialisms can include words (such as "y'all" or "gonna" or "wanna"), phrases (such as "ain't nothin'", "dressed for bear" and "dead as a doornail"), or sometimes even an entire aphorism. ("There's more than one way to skin a cat"). Dictionaries often display colloquial words and phrases with the abbreviation colloq. Colloquialisms are often used primarily within a limited geographical area. An example is for a soft drink. In the upper midwest, it is commonly called "pop", while in other areas it's "soda." In the southern United States, it is commonly called "Coke".

Words that have a formal meaning may also have a colloquial meaning that, while technically incorrect, is recognizable due to common usage. For example, though biweekly is truly defined as "every other week", many dictionaries list both "twice a week" and "every other week".

Auxiliary languages are sometimes assumed to be lacking in colloquialisms, but this varies from one language to another. In Interlingua, the same standards of eligibility apply to colloquialisms as to other terms. Thus, any widely international colloquialism may be used in Interlingua. Expressions such as en las manos de... 'in the hands of...', ¿Qué pasa? 'What's going on?', and ¿Qué diablo? 'What the devil?' 'What the hell?' are common.

chatty, chitchatty, common, communicative, confabulatory, conversational, cozy, everyday, familiar, informal, interlocutory, nonstandard, patois, spoken, substandard, uneducated, unliterary, unstudied, vernacular, vulgar, vulgate