Dye:A coloured substance is known as dye when that can fixed upto the substrate and when fixed is not fugitive e.g. fast to light and not washable by water, dilute acid or alkali. Many natural dyes had been known and they were derived from both vegetable and natural sources. Now a day’s most of the dyes are synthetic and are prepared from aromatic compound. E.g. Coal-tar

Acid dye                               Na          =Na+ +A- (coloured acid)

Basic Dye                             BCl          =B+ (colored)   

Dye or dyestuff: A dye or dyestuff is usually a coloured organic compound or mixture that may be used for imparting colour to a substrate in a reasonably permanent fashion. A dyed substrate should be resistant to a normal laundry or cleansing procedure and stable to light.

Optical brightness or whiteness which has the special properties of absorbing ultraviolet light and re-emitting the visible light to appear the substrate bright is called white dyes. They are colourless compound.

Dyes in a coloured substance but all coloured substance are not dyes. Thus a dye should fix itself on the substrate to give a permanent coloured appearance.