凹陷 (āo xiàn) is a noun meaning 'depression' or 'sunken area'—a physical indentation where the surface dips below the surrounding level. The first character 凹 (āo) means 'concave' or 'hollow', visually resembling a depression; the second character 陷 (xiàn) means 'to sink', 'to submerge', or 'to collapse', often implying downward force or loss of structural integrity. Together, they emphasize a localized, downward deformation caused by pressure, injury, disease, or natural formation.

This term appears frequently in medical contexts (e.g., describing skin indentations after surgery or edema resolution), geology (e.g., sinkholes or craters), engineering (e.g., dents in metal), and everyday descriptions of surfaces like furniture or roads. It carries a neutral-to-clinical tone and is rarely used metaphorically. Unlike general terms like 坑 (kēng, 'pit'), 凹陷 stresses depth relative to adjacent areas and implies an abnormal or notable deviation from flatness or convexity.