First question: A phrase or expression whose meaning cannot be understood in the ordinary sense is an idiom.
Second question: A figure of speech.
Figures of speech are plainly defined as saying one thing in terms of something else. Whenever you say something, but you don't mean it literally, you are using a figure of speech.
Third question: Irony.
Irony is a figure of speech which is a contradiction or incongruity between what is expected and what actually occurs.
Hoped I helped!
