Strictly speaking, chemsex refers to gay or bisexual men using drugs to facilitate sex with other men. Culturally, though, it has become a catch-all term for sex involving drugs and "chemsex parties", where groups of gay and bisexual men meet up, get high, and have sex with one another.

It's important to note that it's distinct from drug use which later leads to sexual activity: chemsex is where men take a certain drug or drugs because they are about to have sex.