A hollow, cavity, dent, or depression—the sunken part of a surface.
駐車場で車のドアに小さな凹みがついていた。
There was a small dent in the car door in the parking lot.
地面のくぼみに水がたまっている。
Water has collected in the depression in the ground.
彼の表情にはどこか凹みが見えたが、理由は話してくれなかった。
There was a certain slump in his expression, but he wouldn't say why.