Colloquialism - a word or phrase used in an easy, informal style of writing or speaking. It is usually more appropriate in speech than formal writing. Colloquialisms appear often in literature since they provide a sense of actual conversation and use the pronunciation, grammar, and vocabulary of everyday speech.
The word is taken from the Latin colloqui, which is a joining of com, meaning “with or together,” and loqui, meaning “to speak” and “conversation.”
Mark Twain makes use of colloquialisms in his Huckleberry Finn, such as in the opening line of the story:
“You don’t know about me without you have read a book by the name of The Adventures of Tom Sawyer, but that ain’t no matter.”
see: dialogue, idiom