An echo question is a question that seeks to confirm or clarify another speaker's utterance (the stimulus), by repeating it back in some form. For example:
A: I'm moving to Greenland. B: You're moving where?
In English, echo questions have a distinctive prosody, featuring a rising intonation. A speaker may use an echo question to seek confirmation because they find the stimulus surprising, or simply because they did not hear it clearly. Echo questions have unusual syntactic properties (including a lack of wh-movement), which have made them a challenge to account for in linguistic theories of questions.