A "birthmark" is a tattoo of the phrase "I am ___", the blank being filled by a person's name. It goes on the small of the back, and is used to confirm a person's true identity in case they are wiped or imprinted.
Origins
Introduced in the episode Epitaph One, Priya Tsetsang has the Active Charlie give her a birthmark. It is said by the writers of the episode that she was the first of the Actives to go above ground and spread the idea of birthmarking among actuals. All of the Actuals in Epitaph One have birthmarks, although Iris gets hers in the course of the episode without actually being an Actual. She was discovered by Zone to be printed when he checked the birthmark of her supposed father, Mr. Miller, who turned out not to be an Actual with that name prior to being a dumbshow.