Tattoo

A tattoo, was a mark or design that was placed on the skin.

The newly regenerated Third Doctor sported a tattoo on one arm. (DW: Spearhead from Space) This was the mark made by the Time Lords to signify the Doctor was an exile. (NA: Christmas on a Rational Planet)

Astrolabus had the star charts he stole tattooed onto his body. (DWM: Once Upon a Time Lord)

The signature emblem of the Corsair was a classical depiction of the mythological Ouroboros, a snake devouring its tail. According to the 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. (DW: The Doctor's Wife)