相 means 'mutual' or 'each other', and 反 means 'to reverse' or 'opposite'. Together, 相反 literally means 'mutually opposite' and functions primarily as an adverb meaning 'on the contrary', 'conversely', or 'in contrast'. It is commonly used to introduce a statement that contradicts or stands in opposition to what was just said — often appearing at the beginning of a clause or sentence. It frequently follows conjunctions like 但是 (but) or 而 (while), and is especially common in logical comparisons, arguments, and emotional contrasts (e.g., expectations vs. reality).

While it can occasionally function as a noun ('the opposite'), its dominant usage is adverbial or conjunctive. It is neutral in register — appropriate in both spoken and written Chinese — but rarely appears alone without context; it almost always connects two contrasting ideas. Learners should avoid confusing it with 反而 (fǎn'ér), which expresses unexpected reversal, or with 矛盾 (máodùn), which means 'contradiction' as a noun.