A colloquialism is a word or phrase used in informal or familiar written and spoken communication. Colloquialisms can be abbreviations of formally recognized words, like “gonna” instead of “going to.” They can also consist of figurative language, like the use of the phrase “hit the road” to mean leaving. In literature, writers use colloquialisms to make characters’ voices more authentic to their identities. 

The term colloquialism derives from the Latin word colloqui (“to converse”), from com (“together”) and loqui (“to speak”). 

In Harper Lee’s novel To Kill a Mockingbird, Jem Finch’s description of Boo Radley is characterized by distinctive Southern colloquialisms:

He goes out, all right, when it’s pitch dark. Miss Stephanie Crawford said she woke up in the middle of the night one time and saw him looking straight through the window at her…said his head was like a skull lookin’ at her. Ain’t you ever waked up at night and heard him, Dill?...Why do you think Miss Rachel locks up so tight at night? I’ve seen his tracks in our back yard many a mornin’, and one night I heard him scratching on the back screen, but he was gone time Atticus got there. 

(Chapter 1)

In this passage, the colloquialisms that stand out include the simile ("like a skull lookin' at her), the contraction "ain't," and truncated words such as "mornin'."

see: dialect