凹 （オウ） concave, hollow, sunken

凹凸 （オウトツ） unevenness, bumpiness, roughness, ruggedness, imbalance, inequality, unevenness, disparity

肋凹 （ロクオウ） costal fovea

窪む （くぼむ） to cave in, to become depressed, to sink

凹む （へこむ） to be dented, to be indented, to yield, to give, to sink, to collapse, to cave in, to be beaten, to be overwhelmed, to yield, to give in, to give up, to be disheartened, to feel down, to feel depressed, to suffer a loss, to lose

穴ぼこ （あなぼこ） hole, hollow, pothole (road, pavement, etc.)