HSK 7-9
凹
āo形
concave; depressed; sunken; indented; female (connector etc)
这块木板中间有点凹。
Zhè kuài mù bǎn zhōng jiān yǒu diǎn āo.
This board is slightly sunken in the middle.
Say it
Hold the button, say it, then let go.
Also free:
Appears across levels (7-9) in the HSK 3.0 word list.