An ELT Glossary : Ellipsis

Definition : The omission of one or more words from a sentence or utterance because they can be understood from the linguistic or situational context.

Examples :

- in Want a biscuit? the words Do you are ellipted. The intonation of the utterance makes it clear that a question is intended, so that grammatical marking becomes redundant.

- in  He got up and walked out  the word he is ellipted in the second clause. The listener/reader's knowledge of grammar tells them that if a verb in a second co-ordinated clause has no explicit subject, it will be the same as that in the first clause.