Ink is a liquid or paste that contains pigments or dyes and is used to colour a surface to produce an image, text, or design. Ink is used for drawing or writing with a pen, brush, or quill. Thicker inks, in paste form, are used extensively in letterpress and lithographic printing.

Ink can be a complex medium, composed of solvents, pigments, dyes, resins, lubricants, solubilisers, surfactants, particulate matter, fluorescents, and other materials. The components of inks serve many purposes; the ink’s carrier, colourants, and other additives affect the flow and thickness of the ink and its dry appearance.

https://en.wikipedia.org/wiki/Ink