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)
In 2050 June Turner was trapped in a nightmare caused by a Bodach. Jorjie Turner entered the nightmare of her mother. She made June think that she had a tattoo of Darius Pike. This caused June to wake up, which made her and K9 able to defeat the Bodach. When she realised that Jorjie didn't have the tattoo June was relieved. (TV: Dream-Eaters)