Words and phrases occurring primarily in speech and informal writing that seeks a relaxed, conversational tone. "
My favorite chow is a burger and a shake" or "
This math exam has me wired" may be acceptable in talking to a roommate, in corresponding with a friend, or in writing a humorous essay for
general readers. Such choices of words, however, would be out of place in formal writing--in, say, a laboratory report or a
letter to your senator. Contractions (let's, don't, we'll) and abbreviated words (photo,
sales rep, TV) are shorthand of spoken language. "
Good writers" use such expressions with an awareness that they produce an effect of casualness.