苦行 refers to ascetic practices or self-discipline, often associated with religious or spiritual traditions (e.g., Buddhism, Hinduism) where practitioners endure physical hardship — such as fasting, celibacy, or prolonged meditation — to achieve spiritual goals. It's a noun, so you'll see it in phrases like 修苦行 ('practice asceticism') or 苦行僧 ('ascetic monk'). The concept carries a tone of voluntary suffering for a higher purpose, not mere hardship.