sesamoid bone
Anatomical term for a small bone embedded within a tendon, such as the patella (kneecap).
膝蓋骨は人体で最大の種子骨である。
The patella is the largest sesamoid bone in the human body.
From 種子 (shushi, 'seed') + 骨 (kotsu, 'bone'), referring to the small, seed-like shape of such bones.