noun
buttocks; gluteal region
Formal anatomical term; in everyday conversation, お尻 (おしり) is more common.
臀部の筋肉を鍛える運動をしています。
I do exercises to strengthen the gluteal muscles.
医者は患者の臀部に注射を打った。
The doctor gave the patient an injection in the buttock.
From Chinese 臀部 (túnbù), meaning 'buttocks'. The kanji 臀 means 'buttocks' and 部 means 'part' or 'region'.