concave

1. Origins, structure, and basic meaning

• Structure: 凹 is a five-stroke character with a recessed middle; its on-reading is おう.
• Basic usage: It describes a dented surface and, figuratively, a discouraged state of mind.

2. Common pitfalls and similar expressions

• A dent and discouragement
  • 凹む(へこむ) can mean a car door becoming dented or a person feeling discouraged after a failure. Distinguish physical deformation from mood.
• The order of the two shapes
  • 凹凸(おうとつ) means unevenness with hollows and bumps. It describes a broader surface condition than the recess indicated by 凹 alone.