Colloquialism (also called colloquial language, colloquial speech, everyday language, or general parlance) is the linguistic style used for casual and informal communication. It is the most common form of speech in conversation among persons in friendship, familial, intimate, and other informal contexts. Colloquialism is characterized by the usage of figurative language, contractions, filler words, interjections, and other informalities such as slang.

In contrast to formal and professional communications, colloquial speech does not adhere to grammar and syntax rules and this may be considered inappropriate and impolite in situations and settings where etiquette is expected or required. It has a rapidly changing lexicon and can also be distinguished by its usage of formulations with incomplete logical and syntactic ordering.

Colloquialism is distinct from formal speech or formal writing. It is the form of language that speakers typically use when they are relaxed and disregarding diction. An expression is labeled colloq. for "colloquial" in dictionaries when a different expression is preferred in formal usage, but this does not mean that the colloquial expression is necessarily slang or non-standard.

Some colloquial language contains a great deal of slang, but some contains no slang at all. Slang is often used in colloquial speech, but this particular register is restricted to particular in-groups, and it is not a necessary element of colloquialism. Other examples of colloquial usage in English include contractions or profanity.

"Colloquial" should also be distinguished from "non-standard". The difference between standard and non-standard is not necessarily connected to the difference between formal and colloquial. Formal, colloquial, and vulgar language are more a matter of stylistic variation and diction, rather than of the standard and non-standard dichotomy. The term "colloquial" is also equated with "non-standard" at times, in certain contexts and terminological conventions.

In the philosophy of language, "colloquial language" is ordinary natural language, as distinct from specialized forms used in logic or other areas of philosophy. In the field of logical atomism, meaning is evaluated in a different way than with more formal propositions.

A colloquial name or familiar name is a name or term commonly used to identify a person or thing in non-specialist language, in place of another usually more formal or technical name. They are not slang nor vulgar, but are informal. This type is speech is used broadly for a variety of topics.

In biology, colloquial names are called "common names". Common pests often have common names. For example, armadillidiidae are often called "rollie pollies", while fly larvae are frequently called "maggots".