What is the term for the omission of a word or phrase that is needed for proper syntax?

The term referring to the omission of a word or phrase that is needed for proper syntax is ellipsis. In grammatical contexts, ellipsis allows for the understanding of a sentence without stating every word explicitly. It relies on the reader or listener to infer the missing parts based on the context. For example, in the sentence "I went to the store, and she did too," the verb "go" is implied in the second clause but is omitted.

In contrast, the other terms do not pertain to omissions in syntax. Pathos relates to emotional appeal in rhetoric, antithesis refers to contrasting ideas presented in parallel structures, and conceit indicates an extended metaphor that draws a striking parallel between seemingly unrelated things. Knowing this, one can accurately identify that ellipsis is the correct term for the specific grammatical omission described in the question.