In the synth context, a modification of an audio signal that is usually applied after the circuitry that originally formed the signal. This is best understood by considering a polyphonic synth, where an effect is a circuit that is positioned at a point in the signal chain after all of the individual voices have been mixed. This may be either inside the synth, or a self-contained unit. The most common effects are reverb, flanging, chorusing, doubling, echo, tremolo, vibrato, phase shifting, and distortion of various types.
The use of reverberation, the oldest known effect in music, predates the electrical age. From the Middle Ages, spaces in which music was to be performed, or a pipe organ installed, were often designed and built so as to create large amounts of reverberation, as performers and audiences figured out that doing so enhanced the music. When radio broadcasting began in the 1910s, most radio studios were located in large cities, and noise from the street and other areas in the building in which the studio was located was an issue. To avoid having such noise in the broadcast, distracting the listener, radio stations found it necessary to heavily insulate the studio space. However, this tended to kill any natural reverberation that the space had, and then listeners complained that the "dry" sound of the broadcast sounded unnatural. So studios began constructing "echo chambers" (a misnomer, since they were designed to avoid discrete echoes). An echo chamber usually consisted of a room in the basement with concrete walls, with no two surfaces parallel. A loudspeaker played the audio to be treated, and microphones in the room picked up the reverberated sound.
In the 1950s, several new approaches arose to adding both reverb and echo. It was discovered that, by injecting sound signals into solid metal objects and then picking up the resulting vibrations from the other side or end of the object with a piezoelectric pickup, reverb could be produced. This led to the common spring reverb, found in most guitar amps since the mid-1950s, as well as some higher-end approaches used in studio equipment, such as plate reverbs. A completely different approach consisted of using a loop of magnetic recording tape as a delay line. Such a device consisted of a record head that recorded a signal onto a loop of tape passing by, followed by a line of playback heads (4-8 usually) that picked up the recorded signal. The speed that the tape moved at, and the distance between the heads, created a mix of delays of different times, which were then mixed together. This approach did not do as good a job of mimicking the multiple surface reflections of a naturally reverberant space, and tended to produce more of a "slapback" sound. Effects designers tried a variety of methods to combat this, sometimes involving multiple combinations of record and playback heads on different tape loops. There were also variations on the theme, such as the Binson Echorec devices that used magnetically-coated cylinders instead of tape, and some devices that relied on a mercury delay line (a technology from the 1950s computer industry, but often too expensive for all but the largest studios).
A revolution in delay-based effects began in the mid-1970s, with the creation of the first bucket brigade integrated circuts. These allowed the creation of delay lines with a large range of delay times, ranging from short enough to produce flanging (a few milliseconds), to chorusing, doubling, and echo effects. These still suffered as reverb-producing devices, though, having the same problems as the tape-based delays. Around 1980, digital delays began to supplant the analog circuits, and these eventually led to more sophisticated processing algorithms that did a much better job of mimicking natural reverb, as well as producing a variety of "artificial" spaces. Beginning in the 1990s, convolution reverb devices made it possible to capture the characteristics of real spaces, producing effects equal to, and in some cases better than, the old echo chambers.
Commonly seen non-delay based effects include tremolo, vibrato, phase shifting, and distortion. The first two, tremolo and vibrato, have both appeared as effects built into guitar amps, as well as studio-based devices for treating other instruments and vocals. However, with a synth, these effects are usually better produced as a part of a patch, by feeding a LFO signal to a VCF or VCO respectively, and so these separate effects are seldom used with synths. Phase shifting is an effect that relies on the natural tendency of filter circuits to produce a phase change in its output that is dependent on the frequency of the input sound. When this is mixed with the unmodified signal, it produces a characteristic set of phase cancellations that depends on the frequencies dependent in the input signal, and how close the filter's cutoff frequency is positioned near these frequencies. Since it is usually not desired in a phase shifter to actually filter the sound, the phase shifter uses an allpass filter, which has its cutoff frequency above the audio range.
Thousands of distortion effects have been produced over the years. Although most of these have been produced for guitarists, they often see use by synth players. Some synths have included distortion circuits or algorithms which are more general in applicability and so are more suitable for synths.
Finally, a number of electromechanical effects have been produced over the years. Two remain in common use; interestingly, they are both associated with the Hammond organ (itself an electromechanical device). The first is the Hammond vibrato scanner, which is built into most Hammond organs and has been produced as a separate effect by various parties. The vibrato scanner consists of series of capacitors of different values, arranged in a circle. A plate attached to a rotor on a shaft serves as one plate for each capacitor; the input signal is fed to the rotor, and it engages the different capacitors in a sequence as it rotates. The effect is somewhat similar to the allpass filter in a phase shifter. The vibrato scanner can produce both vibrato and chorusing effects. The second device is the rotating speaker. This consists of a loudspeaker, or a pair split into a treble and bass unit. Each is inside of, or attached to, an element that rotates and only allows sound to emit from one end or side. The rotary speaker produces a complex mix of frequency modulation, amplitude modulation, and phase shifting effects. Further, most have motors capable of two (or more) speeds; speed changing is subject to the inertia of the rotating elements and so produces a number of additional transient effects while the motor is speeding up or slowing down. All of this has proven difficult to emulate electronically, and so even though there are rotary-speaker emulators available, many performers still prefer the real thing.