Birthmark
A birthmark was a discoloration on the skin, which was present at birth or appeared shortly afterwards. 

Hoshi Sato had a birthmark on her nose. In 2152, she believed that using the transporter had moved it a centimeter lower than it had been previously. Doctor Phlox commented that the subcutaneous pigmentation looked lovely in its new location. She responded to his flippant remark by wondering aloud, "If that machine could move a birthmark, who knows what else it could do?" ()

Charles Tucker III had a birthmark on his right side; in 2153, this birthmark was shared by Sim. ()

Roberta Lincoln had two birthmarks – a small mole on her left shoulder and a somewhat larger, star-shaped mark on a portion of her anatomy she preferred not to discuss in public. ()

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  in 2368. ()

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Anatomy