A deodorant is a substance applied to the body to prevent or mask body odor caused by bacterial breakdown of perspiration, such as that in the armpits, groin, or feet. A subclass of deodorants called antiperspirants prevents sweating itself, typically by blocking sweat glands. Antiperspirants are used on a wider range of body parts at any place where sweat would be inconvenient or unsafe. Other types of deodorant allow sweating but prevent bacterial action on sweat.

The first commercial deodorant, Mum, was introduced and patented in the late nineteenth century by Edna Murphey, an inventor in Philadelphia, Pennsylvania. The product was briefly withdrawn from the market in the US. The modern formulation of the antiperspirant was patented by Jules Montenier on January 28, 1941. This formulation was first found in "Stopette" deodorant spray, which Time magazine called "the best-selling deodorant of the early 1950s".

Use of deodorant with aluminium compounds has been suspected of being linked to breast cancer, but research has not proven any such link.

The human body produces perspiration (sweat) via two types of sweat gland: eccrine sweat glands which cover much of the skin and produce watery odourless sweat, and apocrine sweat glands in the armpits and groin, which produce a more oily "heavy" sweat containing a proportion of waste proteins, fatty acids and carbohydrates, that can be metabolized by bacteria to produce compounds that cause body odor. In addition, the vagina produces secretions which are not a form of sweat but may be undesired and also masked with deodorants.

Human perspiration of all types is largely odorless until its organic components are fermented by bacteria that thrive in hot, humid environments. The human underarm is among the most consistently warm areas on the surface of the human body, and sweat glands readily provide moisture containing a fraction of organic matter, which when excreted, has a vital cooling effect. When adult armpits are washed with alkaline pH soap, the skin loses its protective acid mantle (pH 4.5–6), raising the skin pH and disrupting the skin barrier. Many bacteria are adapted to the slightly alkaline environment within the human body, so they can thrive within this elevated pH environment. This makes the skin more than usually susceptible to bacterial colonization. Bacteria on the skin feed on the waste proteins and fatty acids in the sweat from the apocrine glands and on dead skin and hair cells, releasing trans-3-methyl-2-hexenoic acid in their waste, which is the primary cause of body odor.

Underarm hair wicks the moisture away from the skin and aids in keeping the skin dry enough to prevent or diminish bacterial colonization. The hair is less susceptible to bacterial growth and therefore reduces bacterial odor. The apocrine sweat glands are inactive until puberty, which is why body odor often only becomes noticeable at that time.

Deodorant products work in one of two ways – by preventing sweat from occurring, or by allowing it to occur but preventing bacterial activity that decomposes sweat on the skin.^{[citation needed]}

In 1888, the first modern commercial deodorant, Mum, was developed and patented by a U.S. inventor in Philadelphia, Pennsylvania, Edna Murphey; the small company was bought by Bristol-Myers in 1931. In the late 1940s, Helen Barnett Diserens developed an underarm applicator based on the newly invented ball-point pen. In 1952, the company began marketing the product under the name Ban Roll-On. The product was briefly withdrawn from the market in the U.S., but it is once again available at retailers in the U.S. under the brand Ban. In the UK it is sold under the names Mum Solid and Mum Pump Spray. Chattem acquired the Ban deodorant brand in 1998 and subsequently sold it to Kao Corporation in 2000.