The kanji 凹 represents a "concave" or "indented" shape. It depicts a surface that is sunken or has a depression. Think of a dip or hollow.

・On-yomi (音読み): オウ (ō) ・Kun-yomi (訓読み): くぼ（む） (kubo(mu)) - to be dented; to be sunken.

⚫︎ 凹凸 (おうとつ) Translation: Unevenness; irregularities;凹凸 refers to the overall concave and convex. ⚫︎ 凹面鏡 (おうめんきょう) Translation: Concave mirror. ⚫︎ 凹地 (おうち) Translation: A hollow; a depression.

The kanji 凹 is a pictograph. It visually represents an indentation or a dip in a surface. The two sides indicate the edges, and the bottom part represents the hollow. It's a very straightforward and descriptive character.