Assure is a verb that means to guarantee, promise, or convince someone of something. There are several synonyms for the word "assure". Some common synonyms include affirm, confirm, certify, guarantee, promise, and reassure. Another synonym for "assure" is assert, which means to state strongly or confidently. When someone uses the word "reassure", they mean to provide comfort or relief to someone who is worried or unsure about something. "Certify" implies that someone is testifying to the truth or validity of something. And "guarantee" suggests that something is backed by a promise or warranty. Overall, these synonyms for "assure" all convey the idea of making someone feel more confident or certain about something.