Impotence, also known as erectile dysfunction, is the inability to achieve or maintain an erection long enough to engage in intercourse. Under normal circumstances, when a man is stimulated, his brain sends a message down the spinal cord and into the nerves of the male parts.

As the parts becomes engorged with blood, it enlarges and stiffens, causing an erection. Problems with blood vessels, nerves, or tissues of the parts can interfere with an erection.   More detail...