A tattoo is a form of body modification, made by inserting indelible ink into the dermis layer of the skin to change the pigment. The word tattoo, or tattow in the 18th century, is a loanword from the Polynesian word tatau, meaning, "correct, workmanlike". Before the importation of the Polynesian word, the practice of tattooing had been described in the West as pricking, painting, or staining. We can distinguish five types of tattoos:
1. Traumatic tattoos - According to George Orwell, coal miners could develop characteristic tattoos owing to coal dust getting into wounds. This can also occur with substances like gunpowder. Similarly, a traumatic tattoo occurs when a substance such as asphalt is rubbed into a wound as the result of some kind of accident or trauma. These are particularly difficult to remove as they tend to be spread across several layers of skin, and scarring or permanent discoloration is almost unavoidable depending on the location. An amalgam tattoo is when amalgam particles are implanted in to the soft tissues of the mouth, usually the gums, during dental filling placement or removal; another example of such accidental tattoos is the result of a deliberate or accidental stabbing with a pencil or pen, leaving graphite or ink beneath the skin.
2. "amateur" tattoos - serve as rites of passage, marks of status and rank, symbols of religious and spiritual devotion, decorations for bravery, sexual lures and marks of fertility, pledges of love, punishment, amulets and talismans, protection, and as the marks of outcasts, slaves and convicts. The symbolism and impact of tattoos varies in different places and cultures. (Maori, biblical, ID tattoo from Auschwitz)
3. "Professional" tattoos - done by a (western) tattoo artist in a shop or studio.
4. Cosmetical tattoos - as a part of reconstructive surgery. Or as permanent make-up.
5. Medical tattoo - Medical tattoos are used to ensure instruments are properly located for repeated application of radiotherapy. Tattoos were probably also used in ancient medicine as part of the treatment of the patient (found on mummies and Utzi).
Source: https://en.wikipedia.org/wiki/Tattoo (derived 1-4-2016)