erection /irek″shən/ [L, erigere, to erect] , the condition of hardness, swelling, and elevation observed in the penis and to a lesser degree in the clitoris, usually caused by sexual arousal but also occurring during sleep or after physical stimulation. It results when additional blood enters the organ and blood pressure within the organ increases, and it is influenced by psychic and nerve stimulation. Erection enables the penis to enter the vagina and to emit semen. See also ejaculation, nocturnal emission, priapism.
