A birthmark is one of two types of benign congenital discolourations on the skin. They're caused by either an excess of pigment or an abnormal growth of blood vessels. Pigment birthmarks tend to be genetic in origin, but blood vessel birthmarks are not.
Birthmarks are generally harmless, but may be removed for cosmetic purposes.
On the series, Gregory House thought Thomas Bell was his biological father because they shared a pair of birthmarks - one on the shoulder and the other on the penis.