ELLIPSIS. A clause, or a part of a clause, or a part (usually including the lexical element) of a verbal or nominal group, may be presupposed at a subsequent place in the text by the device of positive omission – that is, by saying nothing, where something is required to make up the sense.
- Halliday (1985), a pag.288

Ellipsis (including substitution) is a relationship involving a particular form of wording, either a clause or some smaller item; it is usually confined to closely contiguous passages, and is particularly characteristic of question + answer or similar ʻadjacency pairsʼ in dialogue.
- Halliday (1985), a pag.289

Another form of anaphoric cohesion in the text is achieved by ellipsis, where we presuppose something by means of what is left out. Like all cohesive agencies, ellipsis contributes to the semantic structure of the discourse. But unlike reference, which is itself a semantic relation, ellipsis sets up a relationship that is not semantic but lexicogrammatical – a relationship in the wording rather than directly in the meaning. For example, in Why didn’t you lead a spade? - I hadn’t got any. the listener has to supply the word 'spades' in order to make sense of the answer.
- Halliday (1985), a pag.296-297