A tattoo is a form of body modification where a design is made by inserting ink, dyes and pigments, either indelible or temporary, into the dermis layer of the skin to change the pigment. Tattoos fall into three broad categories: purely decorative (with no specific meaning); symbolic (with a specific meaning pertinent to the wearer); pictorial (a depiction of a specific person or item)
The word tattoo, or tattow in the 18th century, is a loanword from the Polynesian word tatau, meaning "to write". The Oxford English Dictionary gives the etymology of tattoo as "In 18th c. tattaow, tattow. From Polynesian (Samoan, Tahitian, Tongan, etc.) tatau. In Marquesan, tatu." Before the importation of the Polynesian word, the practice of tattooing had been described in the West as painting, scarring, or staining.
Here's a Tatto: