Compound that can be applied to textiles (or to such other materials as leather or the gelatine in a photographic emulsion) with the effect of changing their colour more or less permanently. Not every strongly coloured compound will behave as a dye: the term is used of a substance that can be dissolved and will then migrate from its solution to a fibre, remaining there when the fibre is rinsed free from excess dye. Ideally, it should survive subsequent washing, exposure to light and rubbing without change in colour. If, in addition, it resists the effects of moderately acidic and alkaline liquids, which are common hazards in normal life, it is a good, fast dye. Unfortunately, the dyes used in the past, and most that are used in the late 20th century, are less resistant than the textiles to which they are applied, so care is needed to avoid damaging them.