The word "therest" has a number of synonyms that can be used depending on the context of the sentence. Some common alternatives include "the remaining," "the rest of," "the balance," "the residue," "the remaining portion," and "what's left." These synonyms can be helpful when trying to convey a sense of what is left after something has been taken away or used up. For example, you might use the synonym "the remaining portion" to explain how much of a cake is left after some has been eaten, or you might use "the balance" to describe the leftover funds in a bank account.