Erection
An erection is the stiffening and enlargement of sexually responsive organs or related tissue due to increased blood flow. The best-known example is a penile erection, but the clitoris can also become erect, and nipples may stand erect in response to stimulation or arousal. The term is used in medical, sexual, and everyday language.
An erection can be triggered by sexual arousal, physical stimulation, or occur involuntarily. If it is painful, unusually persistent, or occurs frequently without a clear reason, medical evaluation is advisable. In a BDSM context, an erection may be part of arousal or response, but it is not a reliable sign of consent or pleasure.