noun
one knee
Refers to a single knee, often used in set phrases like 片膝をつく (to kneel on one knee) or 片膝を立てる (to raise one knee).
彼は片膝をついてプロポーズした。
He proposed on one knee.
床に座り、片膝を立てて話を聞いた。
I sat on the floor, raised one knee, and listened.
Compound of 片 (kata, 'one side') + 膝 (hiza, 'knee').