ColloquialismA colloquialism is an informal expression, that is, an expression not used in formal speech or writing. Colloquialisms can include words (such as "gonna" or "grouty"), phrases (such as "ain't nothin'" and "dead as a doornail"), or sometimes even an entire aphorism ("There's more than one way to skin a cat"). Colloquial words and phrases tend to be marked with the abbreviation "colloq." in dictionaries.
In the past and in some quarters, overuse by native speakers of colloquialisms has been and is sometimes regarded as a sign of substandard ability with the language in question. But in the mouth of a non-native speaker, they are sometimes taken as signalling unusual facility with the language.