Curved inward like the inside of a bowl or sphere.
From Latin concavus, from con- (together) + cavus (hollow). The word entered Middle English through Old French, originally describing any hollow or sunken surface.
Your eye is naturally concave—the cornea and lens curve inward to focus light onto the retina. Nature uses this shape constantly because it's structurally efficient and creates focal points.
Complete word intelligence in one call. Free tier — 50 lookups/day.