Colloquialism
A colloquialism 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"). Dictionaries often display colloquial words and phrases with the abbreviation colloq. Colloquialisms are often used primarily within a limited geographical area.
In some areas, overuse of colloquialisms by native speakers is regarded as a sign of substandard ability with the language. However, in the mouth of a non-native speaker, they are sometimes taken as signaling unusual facility with the language as they may be more difficult for non-native speakers to understand.
A colloquialism can sometimes make its way into otherwise formal speech, as a sign that the speaker is comfortable with his or her audience, in contrast to slang, which if used in formal speech is more likely done so consciously for humorous effect.