凸面 (tū miàn) literally means 'protruding surface' — 凸 (tū) means 'convex', 'bulging outward', and 面 (miàn) means 'surface' or 'face'. Together, they form a technical noun referring to any curved surface that curves outward, like the outer side of a sphere or lens. It is commonly used in physics, engineering, optics, and geometry, but also appears in everyday descriptions of objects such as mirrors, lenses, fruits, or leaves with outward curvature.

In contrast to 凹面 (āo miàn, 'concave surface'), 凸面 describes shapes where the center bulges toward the observer. While formal, it appears in science textbooks, product specifications (e.g., convex rearview mirrors), and even botanical descriptions — for instance, certain leaves or seeds have a naturally convex surface that helps shed water or maximize light exposure.