A colloquialism is an expression not used in formal speech, writing or paralinguistics. Colloquialisms are also sometimes referred to collectively as "colloquial language".  Colloquialisms or colloquial language is considered to be characteristic of or only appropriate for casual, ordinary, familiar, or informal conversation rather than formal speech or writing. Dictionaries often display colloquial words and phrases with the abbreviation colloq. as an identifier.
Some examples of informal colloquialisms can include words (such as "y'all" or "gonna" or "wanna"), phrases (such as "ain't nothin'" and "graveyard dead"), or sometimes even an entire aphorism ("There's more than one way to skin a cat").
Colloquialisms are often used primarily within a limited geographical area, known by linguists to spread through normal conversational interaction of a language, though more often now through informal online interaction. A common example given is the regional term used by people when describing a carbonated soft drink. In the Upper Midwestern United States, in common with Canada, it is commonly called "pop", while in other areas, notably the Northeastern and extreme Western United States, it is referred to as "soda". In the Southern United States, it is commonly called "Coke" regardless of brand. Some southerners even refer to soft drinks as "dope." The common belief is that this is an outdated reference to stimulant properties contained in these drinks. In New England it is occasionally called "tonic." In some areas of Scotland it is referred to as "ginger", and confusion over whether this term referred to all soft drinks or just ginger beer was apparent in the case of Donoghue v Stevenson. (See: Names for soft drinks for more regional examples of colloquial names given to soft drinks.)
Another example of colloquialism is the two different terms for rectangular maple doughnuts. They are called Long Johns in most of the United States, but in the Pacific Northwest (such as Oregon and Washington), they are referred to as Maple bars. In New England (such as Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut), they are called Bear Claws.
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?', are common.
Some linguists make a distinction between colloquialisms and slangisms (slang words). According to linguist Ghil'ad Zuckermann, "slang refers to informal (and often transient) lexical items used by a specific social group, for instance teenagers, soldiers, prisoners, or thieves. Slang is not considered the same as colloquial (speech), which is informal, relaxed speech used on occasion by any speaker; this might include contractions such as you’re, as well as colloquialisms. A colloquialism is a lexical item used in informal speech; whilst the broadest sense of the term ‘colloquialism’ might include slangism, its narrow sense does not. Slangisms are often used in colloquial speech but not all colloquialisms are slangisms. One method of distinguishing between a slangism and a colloquialism is to ask whether most native speakers know the word (and use it); if they do, it is a colloquialism. However, the problem is that this is not a discrete, quantized system but a continuum. Although the majority of slangisms are ephemeral and often supplanted by new ones, some gain non-slang colloquial status (e.g. English silly – cf. German selig ‘blessed’, Middle High German sælde ‘bliss, luck’ and Zelda, a Jewish female first name) and even formal status (e.g. English mob)."