Stereophonic sound, commonly called stereo, is the reproduction of sound, using two or more independent audio channels, through a symmetrical configuration of loudspeakers, in such a way as to create a pleasant and natural impression of sound heard from various directions, as in natural hearing. It is often contrasted with monophonic (or "monaural", or just mono) sound, where audio is in the form of one channel, often centered in the sound field (analogous to a visual field).

The word "stereophonic" — derived from Greek stereos = "solid" and phōnē = "sound" — was coined by Western Electric, by analogy with the word "stereoscopic".

In popular usage, stereo usually means 2-channel sound recording and sound reproduction using data for more than one speaker simultaneously.

In technical usage, stereo or stereophony means sound recording and sound reproduction that uses stereographic projection to encode the relative positions of objects and events recorded. A stereo system can include any number of channels, such as the surround sound 5.1- and 6.1-channel systems used on high-end film and television productions. However, in common use it refers to systems with only two channels.

The electronic device for playing back stereo sound is often referred to as "a stereo".

During two-channel stereo recording, two microphones are placed in strategically chosen locations relative to the sound source, with both recording simultaneously. The two recorded channels will be similar, but each will have distinct time-of-arrival and sound-pressure-level information. During playback, the listener's brain uses those subtle differences in timing and sound-level to triangulate the positions of the recorded objects.

Stereo recordings often cannot be played on monaural systems without a significant loss of fidelity. Since each microphone records each wavefront at a slightly different time, the wavefronts are out of phase; as a result, constructive and destructive interference can occur, if both tracks are played back on the same speaker. This phenomenon is known as phase cancellation.

This phenomenon has actually been used to effect on the track Jenny Ondioline by the band Stereolab on their album Transient Random-Noise Bursts With Announcements. This track, when played back is reproduced in stereophonic sound until the moment in time 13'35" when a voice announces "The recorded signal is recorded equally on both channels, but is out of phase." After this announcement, the music becomes destructive, chaotic and distorted and is reproduced as a monaural signal.

Some traditional music genres, e.g. Andean music, require stereo recording strictly to make adequate representation of its dualistic nature. See Siku (panpipe) for explanation.
X-Y technique: intensity stereophony
X-Y Stereo Microphone placement

Here, two directional microphones at the same place, and typically pointing at an angle 90° or more to each other — see also "The Stereophonic Zoom" by Michael Williams. A stereo effect is achieved through differences in sound pressure level between two microphones. The level difference of 18 dB (16 to 20 dB) is needed for hearing the direction of a loudspeaker. Due to the lack of differences in time-of-arrival / phase-ambiguities, the sonic characteristic of X-Y recordings has less sense of space and depth when compared to recordings employing an AB-setup.

When two figure-of-eight microphones are used, facing ±45° with respect to the sound source, the X-Y-setup is called a Blumlein Pair. The sonic image produced is realistic, almost 'holographic'.
A-B technique: time-of-arrival stereophony
A-B Stereo Microphone placement

This uses two parallel omnidirectional microphones some distance apart, so capturing time-of-arrival stereo information as well as some level (amplitude) difference information, especially if employed in close proximity to the sound source(s). At a distance of about 50 cm (0.5 m) the time delay (time of arrival difference) for a signal reaching first one and then the other microphone from the side is approximately 1.5 msec (1 to 2 msec). According to Eberhard Sengpiel this is enough to locate the sound source exactly at the speaker on the respective side, resulting in a stereophonic pickup angle of 180°. If you increase the distance between the microphones you effectively decrease the pickup angle. At 70 cm distance it is about equivalent to the pickup angle of the near-coincident ORTF-setup. This technique can produce phase issues when the stereo signal is mixed to mono.
M/S technique: Mid/Side stereophony
MId-Side Stereo Microphone technique

This coincident technique employs a bidirectional microphone facing sideways and another microphone (generally a variety of cardioid, although Alan Blumlein described the usage of an omnidirectional transducer in his original patent) at an angle of 90° facing the sound source. The left and right channels are produced through a simple matrix: Left = Mid + Side, Right = Mid - Side (the polarity-reversed side-signal). This configuration produces a completely mono-compatible signal, and if the Mid and Side signals are recorded — rather than the matrixed Left and Right — the stereo width can be manipulated after the recording has taken place, which makes it especially useful for the usage on film-based projects.
Playing back stereo recordings

Stereophonic sound attempts to create an illusion of location for various instruments within the original recording. The recording engineer's goal is usually to create a stereo "image" with localization information. When a stereophonic recording is heard through loudspeaker systems rather than headphones, each ear of course hears sound from both speakers. The audio engineer may and often does use more than two microphones, sometimes many more, and may mix them down to two tracks in ways that exaggerate the separation of the instruments to compensate for the mixture that occurs when listening via speakers.

Descriptions of stereophonic sound tend to stress the ability to localize the position of each instrument in space, but in reality many people listen on playback systems that do a poor job of re-creating a stereo "image". Many listeners assume that "stereo" sound is "richer" or "fuller-sounding" than monophonic sound. This is inaccurate — stereo and mono can have equally detailed abilities to play recorded notes. The spatial illusion is what sets stereo recordings apart from mono recordings.

When playing back stereo recordings, best results are obtained by using two speakers, in front of and equidistant from the listener, with the listener located on the center line between the two speakers.