A disc jockey (also known as disk jockey, DJ or deejay) is a person who selects and plays recorded music for an audience. Originally, disk referred to phonograph records, while disc refers to the Compact Disc, and has become the more common spelling.

There are several types of disc jockeys. Radio DJs introduce and play music that is broadcast on AM, FM, shortwave, digital, or internet radio stations. Club DJs select and play music in bars, nightclubs, discothèques, at raves, or even in a stadium. Hip-hop disc jockeys select and play music using multiple turntables, often to backup one or more MCs, and they may also do turntable scratching to create percussive sounds. In reggae, the disc jockey (deejay) is a vocalist who raps, "toasts", or chats over pre-recorded rhythm tracks while the individual choosing and playing them is referred to as a selector. Mobile disc jockeys travel with portable sound systems and play recorded music at a variety of events.

Equipment and techniques

• Sound recordings in a DJ's preferred medium (e.g., vinyl records, Compact Discs, computer media files, etc.);
• A combination of two devices (or only one, if playback is digital) to play sound recordings, for alternating back and forth to create a continuous playback of music (e.g., record players, Compact Disc players, computer media players such as an MP3 player, etc.);
• A sound system for amplification or broadcasting of the recordings (e.g., portable audio system, PA system) or a radio broadcasting system;
• A DJ mixer, which is an electronic (usually 2- or 4-channel) audio mixer with a crossfader used to smoothly go from one song to another, using two or more playback devices;
• Headphones, used to listen to one recording while the other recording is being played to the audience; and
• Optionally, a microphone, so that the DJ can introduce songs and speak to the audience.

Other equipment could or can be added to the basic DJ setup (above), providing unique sound manipulations. Such devices include, but are not limited to:

• Electronic effects units (delay, reverb, octave, equalizer, chorus, etc.). Some club DJs use a sub-harmonic synthesizer effect which either doubles low frequencies with energy added an octave lower or synthesizes harmonics such that the impression of a very low bass sound is added to the mix.
• A computerised performance system, which can be used with vinyl emulation software to manipulate digital files on the computer in real time.
• Multi-stylus headshells, which allow a DJ to play different grooves of the same record at the same time.
• Special DJ digital controller hardware can manipulate digital files on a PC or laptop;
• Samplers, sequencers, electronic musical keyboards (synthesizers), or drum machines.