Common usage
The motion picture era
The first motion picture to be exhibited with stereophonic sound was Walt Disney's Fantasia, released in November 1940, for which a specialized sound process, Fantasound, was developed. Fantasound used a separate film containing four optical sound tracks. Three of the tracks were audible, and the fourth track controlled the volume level of the theater's amplifiers. The film was not a financial success, however, and after two months of road-show exhibition in selected cities, its soundtrack was remixed into mono sound for general release.
In the early 1940s, the forward thinking Alfred Newman directed the construction of a sound stage equipped for multi channel recording for 20th Century Fox studios. Several soundtracks from this era still exist in their multichannel elements, some of which have been released on cd includiing How Green Was My Valley, Anna and the King of Siam, and The Day the Earth Stood Still.
The advent of magnetic tape recording made high-fidelity synchronized multichannel recording technically straightforward, though costly. Motion picture theatres could afford the cost, and that is where the real introduction of stereophonic sound to the public occurred. Stereo sound first became a wide success with the advent of Cinerama in 1952. Cinerama was a spectacular wide-screen process fully comparable to today's IMAX. Cinerama practically required a specially built theatre for its presentation. It used seven magnetic sound tracks, six of them audible plus a seventh track that controlled the volume level of the amplifiers. The system was developed by Hazard Reeves, a pioneer in magnetic recording technology. By all accounts, including accounts by those who have experienced the process in rare recent showings, the sound was as spectacular as the picture and excellent even by modern standards. The movie industry moved quickly to create simpler and cheaper wide-screen systems, such as CinemaScope, which used up to four magnetic sound tracks, and which were capable of being retrofitted into existing theatres. Cole Porter memorialized the era in a 1954 song:-

If Zanuck's latest picture were the good old-fashioned kind,
There'd be no one in front to look at Marilyn's behind.
If you want to hear applauding hands resound
You've gotta have glorious Technicolor,
Breathtaking Cinemascope and
Stereophonic sound.
Broadcasting in stereo
Radio: The BBC's experimental transmitting station 5XX in Daventry, Northamptonshire, made radio's first stereo broadcast in December 1925, of a concert conducted by Sir Hamilton Harty from Manchester, with 5XX broadcasting the right channel nationally by long wave, and local BBC stations broadcasting the left channel by medium wave. The BBC repeated the experiment in 1926, using 2LO in London and 5XX at Daventry. Following experimental FM stereo transmissions in the London area in 1958, the first regular BBC transmissions using an FM stereo signal began on the BBC's Third Programme network on August 28, 1962.
WMAQ-FM, a Chicago FM radio station, made radio's first commercial stereophonic broadcast on December 17, 1953, with the first show of the series The Northerners, featuring a choral group of the same name. After several years of experimental stereo broadcasts, the Federal Communications Commission licensed regular stereophonic FM radio broadcasting to begin in the United States on June 1, 1961, with WEFM in the Chicago area and WGFM in Schenectady, New York reporting as the first stereo stations.
Television: The entire 1956-1957 season of The Lawrence Welk Show on ABC was broadcast with stereophonic sound in some cities, with one audio channel broadcast via television and the other over the ABC radio network. By the same method, NBC television and the NBC radio network offered stereo sound for The George Gobel Show on October 21, 1958. ABC's Walt Disney Presents made a stereo broadcast of The Peter Tchaikovsky Story, including scenes from Disney's latest animated feature Sleeping Beauty, on January 30, 1959 by using ABC-affiliated AM and FM stations for the left and right audio channels.
Regular network transmission of stereo audio began on NBC in 1984.

In common usage, a "stereo" is a two-channel sound reproduction system, and a "stereo recording" is a two-channel recording. This is a cause for much confusion, since five (or more) channel home theater systems are not popularly described as "stereo". It is worth noting that most film soundtracks are not recorded using stereo techniques, so while capable of stereo playback, most home theater systems rarely do.
Most two-channel recordings are stereo recordings only in this weaker sense. Pop music, in particular, is usually recorded using close miking techniques, which artificially separates signals into several tracks. The separate tracks are then mixed into a two-channel recording which often bears little or no resemblance to the actual physical and spatial relationship of the musicians at the time of the original performance. Indeed, it is not uncommon for different tracks of the same song to be recorded at different times, and even in different studios, and then mixed into a final two-channel recording for commercial release. Classical music recordings are a notable exception.
Balance can mean the amount of signal from each channel reproduced in a stereo audio recording. Typically, a balance control will have 0 dB of gain in the center position for both channels, and attenuate one channel as the control is turned, leaving the other channel at 0 dB.
The phrase "rechanneled stereo" refers to the artificial rechanneling of mono recordings to simulate stereo that was common in the 1950s-1960s. Because of this usage, "stereo" or "in stereo" is sometimes used colloquially for when two, as distinct from one, of something are present.

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