A concave mirror (āo miàn jìng) is a reflective surface that curves inward, like the inside of a spoon. The term breaks down literally: 凹 (āo) means 'concave' or 'sunken', 面 (miàn) means 'surface' or 'face', and 镜 (jìng) means 'mirror'. Together, they describe a mirror whose reflecting surface is curved inward, causing parallel light rays to converge at a focal point. This optical property makes it useful in scientific instruments, headlights, solar concentrators, and makeup mirrors.

In Chinese, 凹面镜 is a formal, technical term used primarily in physics, engineering, and educational contexts. It contrasts with 凸面镜 (tū miàn jìng), the convex mirror. While the characters are straightforward, learners should note that 凹 is pronounced āo (not ào) — a common tone error — and that the word functions exclusively as a noun, never as a verb or adjective.