Depth: the distance from top to bottom or from a surface inward. Can also be used figuratively to indicate the degree, profundity, or complexity of something.
この池の深さは約三メートルだ。
The depth of this pond is about three meters.
問題の深さに気づかなかった。
I didn't realize how deep/serious the problem was.
箱の深さを定規で測った。
I measured the depth of the box with a ruler.