Prostitution is a type of sex work that involves engaging in sexual activity in exchange for payment. The definition of "sexual activity" varies, and is often defined as an activity requiring physical contact with the customer. The requirement of physical contact also creates the risk of transferring infections. Prostitution is sometimes described as sexual services, commercial sex or, colloquially, hooking. It is also described as gender-based violence. It is sometimes referred to euphemistically as "the world's oldest profession" in the English-speaking world. A person who works in the field is usually called a prostitute or sex worker, but other words, such as hooker and whore, are sometimes used pejoratively to refer to those who work in prostitution. The majority of prostitutes are female and have male clients.