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Simulcast is a portmanteau of "simultaneous broadcast", and refers to programs or events broadcast across more than one medium, or more than one service on the same medium, at the same time. For example, Virgin Radio is simulcast on both AM and on satellite radio, and the BBC's Prom concerts are often simulcast on both BBC Radio 3 and BBC Television. Another application is the transmission of the original-language soundtrack of movies or TV series over radio, with the television broadcast having been dubbed into a local language.

Before stereo TV sound transmission was possible, simulcasting on TV and Radio was a method of effectively transmitting "stereo" sound to music TV broadcasts. The first such transmission was in 1974, when the BBC broadcast a recording of Van Morrison's London Rainbow Concert simultaneously on BBC2 TV and Radio 2: see It's Too Late To Stop Now.

Similarly, in the 1980s, before Multichannel Television Sound, or home theater was commonplace in American households, broadcasters would air a high fidelity version of a television program's audio portion over FM stereo simultaneous with the television broadcast. PBS stations were the most likely, especially when airing a live concert. In America, simulcast most often refers to the practice of offering the same programming on an FM and AM station owned by the same entity, in order to cut costs. With the advent of solid state AM transmitters and computers, it has become very easy for AM stations to broadcast a different format without additional cost; ergo, simulcast between FM/AM combos is rarely seen(heard) today. Normally, AM stations broadcast some type of talk format; depending on the population, the format may be ethnic, predominantly Mexican. During Afrikaner rule in South Africa, many programs were dubbed in Afrikaans. The English soundtrack was available on Radio 2000. This could be selected using a button labeled simulcast on many televisions manufactured before 1995.

In another case, popular programs will be aired simultaneously on different services in adjacent countries, such as The Simpsons, airing Sunday evenings at 8:00 p.m. (Eastern and Pacific times) on both Fox in the United States and Global in Canada. "Simulcast" is often a colloquial term for the related Canadian practice of simultaneous substitution.

In sports, simulcasts are when a single announcer broadcasts play-by-play coverage both over television and radio. The practice was common in the early years of television, but since the 1980s, most teams have used a separate team for television and for radio. Chick Hearn and Rod Hundley were the last broadcasters in professional basketball to simulcast, while in baseball, Vin Scully continues to simulcast the first few innings of games. The National Hockey League only has two remaining teams of broadcasters that simulcast: Daryl Reaugh and Ralph Strangis (Dallas) and Rick Jeanneret and Jim Lorentz (Buffalo).

Simulcasts via satellite can be a challenge, as there is a significant delay because of the distance (nearly 50,000 miles or 80,000 km round-trip) involved. Anything involving video compression (and to some extent audio data compression) also has an additional significant delay, which is noticeable when watching local TV stations on direct broadcast satellites. Even though the process is not instantaneous, this is still considered a simulcast because it is not intentionally stored anywhere.

(Multiplexing -- also sometimes called "multicasting" -- is something of a reversal of this situation, where multiple program streams are combined into a single broadcast. The two terms are sometimes confused.)

In horse racing, a simulcast is a broadcast of a horse race which allows wagering at two or more sites; the simulcast often involves the transmission of wagering information to a central site, so that all bettors may bet in the same betting pool, as well as the broadcast of the race.

On cable television systems, analog-digital simulcasting (ADS) means that analog channels are duplicated as digital subchannels. Digital tuners are programmed to use the digital subchannel instead of the analog. This allows for smaller, cheaper, cable boxes by eliminating the analog tuner and some analog circuitry. On DVR's, it eliminates the need for an MPEG encoder to convert the analog signal to digital for recording. The primary advantage is the elimination of interference, and as analog channels are dropped, the ability to put 10 or more SDTV (or 2 HDTV, or various other combinations) channels in its place. The primary drawback is the common problem of over-compression (quantity over quality) resulting in fuzzy pictures and pixelation.

In universities with multiple campuses, simulcasting may be used for a single teacher to teach class to students in two or more locations at the same time, using videoconferencing equipment.

In many public services, radios used by police and fire officials are often simulcasted on multiple frequencies so that other agencies can hear what is going on and what the dispatcher is saying.

• Single Channel Simulcast