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An erection, or penile erection, is a physiological phenomenon where the human penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal or sexual attraction. The angle of an erect penis varies from pointing downwards, upwards or sideways, and the penis may also bend.

Erections during sleep or when waking up are known as nocturnal penile tumescence (NPT). A penis which is partly, but not fully, erect is sometimes known as a semi-erection; a penis which is not erect is typically referred to as being flaccid, or soft.

Terminology
Erections have many informal and slang terms, including 'stiffy', 'hard-on', 'boner' and 'woody'.