A detergent is a compound, or a mixture of compounds, intended to assist cleaning.
Such a substance, especially those made for use with water, may include any of various components having several properties:
- surfactants to "cut" grease and to wet surfaces
- abrasives to scour
- substances to modify pH, either to affect performance or stability of other ingredients, or as caustics to destroy dirt
- water "softeners" to counteract the effect of "hardness" ions on other ingredients
- oxidants (oxidizers) for bleaching and destruction of dirt
- materials other than surfactants to keep dirt in suspension
- enzymes to digest proteins, fats, or carbohydrates in dirt or to modify fabric feel
- ingredients, surfactant or otherwise, modifying the foaming properties of the cleaning surfactants, to either stabilize or counteract foam
plus ingredients having other properties to go along with detergency, such as fabric brighteners, softeners, etc., and colors, perfumes, etc.
Not only the material to be cleaned, but also the apparatus to be used, and type of and tolerance for dirt, dictate vast differences in the compositions of detergents. For instance, to clean glass one might use chromic acid solution (to get it very clean for certain precision-demanding purposes), a high foaming mixture of surfactants with low skin irritation (for hand washing of drink glasses in a sink or dishpan), any of various non-foaming compositions for a dishwashing machine, an ammonia-containing solution for cleaning windows with no rinsing, or yet a different kind of formula for windshield washer fluid for a vehicle in motion.
Sometimes the word "detergent" is used in distinction to "soap". For a while during the infancy of other surfactants as commercial detergent products, the term "syndet", short for "synthetic detergent" was promoted to indicate this, but it hasn't caught on too well, and is incorrect anyway because soap is itself synthesized via saponification of glycerides. The term "soapless soap" also saw a brief vogue. Unfortunately there is no accurate term for detergents not made of soap other than "soapless detergent" or "non-soap detergent".
Also, the term "detergent" is sometimes used for surfactants in general, even when they are not used for cleaning. As can be seen above, this too is terminology that should be avoided as long as the term "surfactant" itself is available.
Technically plain water, if used for cleaning, is a detergent. Probably the most widely used detergents other than water are soaps or mixtures composed chiefly of soaps. However, not all soaps have significant detergency. Often the word "soap" is used to indicate any detergent, especially those that have characteristics similar to those of soap; it's hard to beat a 4-letter word for popularity, even at the cost of precision.