Erection

Three columns of erectile tissue make up most of the volume of the penis.

Erection is defined as:

the condition of becoming rigid and elevated, as erectile tissue when filled with blood.
especially the swelling and rigidity that occur in the two corpora cavernosa and corpus spongiosum of the penis as a result of sexual or other types of stimulation. Impulses received by the nervous system stimulate a flow of blood from the arteries leading to the penis, where the erectile tissue fills with blood, and the penis becomes firm and erect. Erection makes possible the transmission of semen into the body of the female. Erection can also occur to some extent in the clitoris and the nipples of the female.^[1]

Side views and comparison of the stages human penis erection