Masochism
Masochism is deriving pleasure — often sexual — from experiencing pain, discomfort, or humiliation. The masochist wants to receive sensations that would ordinarily be unwelcome (impact, pinching, biting, intense stimulation, or emotional humiliation) and finds them arousing, cathartic, or blissful. It's the 'M' in BDSM's S&M, the natural counterpart to sadism (taking pleasure in giving that pain), and a core current running through much of kink. It ranges from enjoying a firm spanking to seeking intense pain play. The pleasure comes from the sensation and the surrender both.
For many masochists, pain triggers a flood of endorphins and adrenaline that can tip into a floaty, euphoric 'subspace,' while the act of enduring and surrendering is its own deep release. Being able to take pain — and offering that to a partner — can feel powerful, cathartic, and intimate.
A masochist receives the sensations they crave from a partner (often a sadist or dominant) — impact, pinching, clamps, biting, or intense stimulation — calibrated through negotiation and feedback, usually within a wider D/s scene. Many use a pain scale to guide intensity.
Because pain can mask injury and endorphins can push people past their real limits, negotiate intensity and hard limits up front, keep a safeword, and have the top watch for genuine harm rather than just 'good' pain. Follow the safety for each specific activity (impact zones, clamp timing, and so on), and give aftercare, since the post-scene 'drop' can hit hard.