kneecap; patella
Colloquial anatomical term; literally 'knee plate'. Used in everyday conversation.
I fell and hit my kneecap.
If you break your kneecap, you won't be able to walk.
Compound of 膝 (ひざ, 'knee') + の (possessive particle) + 皿 (さら, 'plate'), from the resemblance of the patella to a small plate.