A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism by chemical or biological means. The US Environmental Protection Agency (EPA) uses a slightly different definition for biocides as "a diverse group of poisonous substances including preservatives, insecticides, disinfectants, and pesticides used for the control of organisms that are harmful to human or animal health or that cause damage to natural or manufactured products". When compared, the two definitions roughly imply the same, although the US EPA definition includes plant protection products and some veterinary medicines.

The terms "biocides" and "pesticides" are regularly interchanged, and often confused with "plant protection products". To clarify this, pesticides include both biocides and plant protection products, where the former regards substances for non-food and feed purposes and the latter regards substances for food and feed purposes.

When discussing biocides a distinction should be made between the biocidal active substance and the biocidal product. The biocidal active substances are mostly chemical compounds, but can also be microorganisms (eg bacteria). Biocidal products contain one or more biocidal active substances and may contain other non-active co-formulants that ensure the effectiveness as well as the desired pH , viscosity , colour, odour, etc. of the final product. Biocidal products are available on the market for use by professional and/or non-professional consumers.

Although most of the biocidal active substances have a relative high toxicity, there are also examples of active substances with low toxicity, such as CO 2 , which exhibit their biocidal activity only under certain specific conditions such as in closed systems. In such cases, the biocidal product is the combination of the active substance and the device that ensures the intended biocidal activity, ie suffocation of rodents by CO 2 in a closed system trap. Another example of biocidal products available to consumers are products impregnated with biocides (also called treated articles), such as clothes and wristbands impregnated with insecticides, socks impregnated with antibacterial substances etc.

Biocides are commonly used in medicine , agriculture , forestry , and industry . Biocidal substances and products are also employed as anti-fouling agents or disinfectants under other circumstances: chlorine, for example, is used as a short-life biocide in industrial water treatment but as a disinfectant in swimming pools. Many biocides are synthetic, but there are naturally occurring biocides classified as natural biocides , derived from, eg, bacteria and plants. 

A biocide can be:

A pesticide : this includes fungicides , herbicides , insecticides , algicides , molluscicides , miticides and rodenticides .

An antimicrobial : this includes germicides , antibiotics , antibacterials , antivirals , antifungals , antiprotozoals and antiparasites . See also spermicide .