The term "deep structure" refers to the underlying grammatical structure of a sentence or phrase. Some synonyms for this term might include "underlying structure," "fundamental structure," "core structure," or "essential structure." These terms all refer to the same idea of the basic, hidden structure that governs language use and meaning. They are useful for linguists and language students who are interested in understanding the technical workings of language and grammar. By studying the deep structure of a sentence, they can gain insights into the ways that language is used and how meaning is conveyed.