确信 is a verb meaning 'to be convinced' or 'to believe firmly'. It expresses strong, unwavering confidence in the truth or reality of something — stronger than just thinking or suspecting. The first character 确 (què) means 'certain' or 'definite', and the second character 信 (xìn) means 'to believe' or 'to trust'; together, they form an emphatic compound meaning 'to hold as certain'. This word often appears after subjects expressing people’s mental states — such as 我、他、我们 — and is commonly followed by a clause introduced by that (that-clause), e.g., 确信他不会迟到.

It is frequently used in formal speech, writing, and news reports to convey authoritative conviction — for instance, scientists confirming findings or officials stating policy positions. Unlike the more casual 相信, 确信 implies evidence, experience, or reasoning underpins the belief. It is not used for tentative or hopeful beliefs (e.g., 'I hope he comes' ≠ 我确信他来), nor does it take objects directly like nouns; rather, it governs entire clauses or embedded sentences.