A birthmark was a discoloration on the skin, which was present at birth or appeared shortly afterwards.
Roberta Lincoln had two birthmarks – a small mole on her left shoulder and a somewhat larger, star-shaped mole on a portion of her anatomy she preferred not to discuss in public. (TOS: "Assignment: Earth")
Wesley Crusher had a birthmark somewhere on the lower part of his body. Robin Lefler's Starfleet Academy friends informed her of this, and she teased Wesley about it on his return to the USS Enterprise-D in 2368. (TNG: "The Game")