Dictionary.net

The insertion of permanent colors in the skin by introducing them through punctures. Tattooing of the cornea is the permanent coloring of the cornea chiefly to conceal leukomatous spots. (Dorland, 27th ed.)

The permanent coloring of the skin or of the cornea, chiefly to cover leukomatous spots.

The process of introducing particles of coloring matter into the true skin, by punctures or incisions, so as to produce indelible stains.

n. The practice of pricking the skin and staining the punctures with an indelible dye or colouring matter.