A tattoo was a mark or design that was placed on the skin.
The newly regenerated Third Doctor sported a tattoo on one arm. (TV: Spearhead from Space) This was the mark made by the Time Lords to signify the Doctor was a convict. Similar tattoos were placed on prisoners in Shada; they "snapped" when the prisoners were released. (PROSE: Christmas on a Rational Planet) Daniel Joyce had one of these tattoos. (PROSE: Unnatural History)
The signature emblem of the Corsair was a classical depiction of the mythological Ouroboros, a snake devouring its tail. According to the Eleventh Doctor, he had the emblem tattooed on his body after each regeneration, even when he regenerated as a female. Without it, the Corsair didn't feel like him/herself. In his last regeneration, the tattoo appeared on the inside of his left forearm. (TV: The Doctor's Wife)