From Halopedia, the Halo wiki
A tattoo is a permanent design marked on skin using inked needles. Tattoos are a common style of body modification used among humans. Humanity's interest in body art dates back to their prehistoric civilization, when military officers were adorned with white facial markings very reminiscent of tattoos. Some groups in the ancient civilization's regressed remnants used scarification in lieu of tattoos, with religious meaning attached to the carved markings. Other decorative and noninvasive body markings are also made using body paint or make-up. While facial paint is sometimes used for fashion, it could also be used to mark one's social status or serve as war paint when going into battle.
Tattoos of organizations
Tattoos are often used to promote bonding between members of a certain group.
Tattoos are frequently displayed with pride by the Orbital Drop Shock Troopers. Melissa McKay wore a tattoo of a skull and the letters "ODST" on her bicep. Cadmon Lasky also got a tattoo of the ODST emblem on his back while he was at Jericho VII. Jacob Keyes, while not an ODST, was given an ODST tattoo of the kanji symbol for either "bastard" or "bad ass", as the meaning differs depending on who is asked.
Keepers of the One Freedom
Tattoos can also be personal decorations of individual persons.
List of appearances