Slang refers to a type of language that's too informal to use in certain situations. You can tell a word or phrase is slang when it becomes uncool to use it after a while — like "groovy" or "far out."
Often, slang terms are considered vulgar or offensive to use in polite conversation. However, over time, many slang expressions have become part of our standard vocabulary, as they are more commonly used. As a noun, slang can also mean "the characteristic language of a particular group of people" like a slang used by computer programmers.