A synthesizer (often abbreviated "synth") is an electronic instrument capable of producing sounds by generating electrical signals of different frequencies. These electrical signals are played through a loudspeaker or set of headphones. Synthesizers can usually produce a wide range of sounds, which may either imitate other instruments ("imitative synthesis") or generate new timbres.

Synthesizers use a number of different technologies or programmed algorithms, each with their own strengths and weaknesses. Among the most popular waveform synthesis techniques are subtractive synthesis, additive synthesis, wavetable synthesis, frequency modulation synthesis, phase distortion synthesis, physical modeling synthesis and sample-based synthesis. Other sound synthesis methods, like subharmonic synthesis or granular synthesis, are not found in music synthesizers.

Synthesizers are often controlled with a piano-style keyboard, leading such instruments to be referred to simply as "keyboards". Several other forms of controller have been devised to resemble violins, guitars (see guitar synthesizer) and wind-instruments. Synthesizers without controllers are often called "modules", and they can be controlled using MIDI or CV/Gate methods.