Tumescence

Tumescence is the quality or state of being tumescent or swollen. Tumescence usually refers to the normal engorgement with blood (vascular congestion) of the erectile tissues, marking sexual excitation and possible readiness for sexual activity. The tumescent sexual organ in men is the penis and in women is the clitoris. Detumescence is the reversal of this process, by which blood leaves the erectile tissue, returning to the flaccid state.

Regularly, men who experience erectile dysfunction are given a nocturnal penile tumescence (NPT) test, usually over a three-day period. Such a test detects the presence of an erection occurring during sleep using either:

The goal of nocturnal penile tumescence testing is to determine whether a man can experience an erection while sleeping after reporting he is unable to experience an erection while awake. If a man does obtain an erection while sleeping, but cannot obtain one while awake, a psychological cause or a medication side effect is usually suspected. Otherwise, if a man does not obtain an erection in either state, a physiological cause is usually suspected.

...
Wikipedia