A dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied, or a material that imparts colour to other substances (eg food, fabric, skin, paints) thus making them (normally) more attractive or distinctive.
The dye is usually used as an aqueous solution and may require a mordant to improve the fastness of the dye on the fibre. (In contrast, a pigment generally has no affinity for the substrate, and is insoluble)
Dyes are obtained from either animal, vegetable or mineral origin with no or very little processing. By far the greatest source of dyes has been from the plant kingdom, notably roots, berries, bark, leaves and wood, but only a few have ever been used on a commercial scale.