Concave is a Chinese character, a pictographic character, and the pinyin is āo, wā (wā is cleared in the dictionary). Concave has two meanings, namely high around and low in the middle, which is opposite to "convex"; it means the same as "wa" (mostly used in place names).

The word "口" is pronounced āo,, as in "dent". It is only pronounced wā in some place names.

"Tang Yun" Wuqiaqie "Jiyun" "Yunhui" Yiqiaqie,

◎High around, low in the middle, opposite to "convex": ~convex and uneven. ~Lens. ~Block printing (one of the printing techniques, as opposed to "relief printing").