1. An electronic musical instrument, usually operated by keyboard and pedals, in which sounds are produced by, for example, additive, subtractive, or FM synthesis, by sampling acoustic waveforms, or by other modelling techniques. See also attack decay sustain release.

2. A laboratory instrument for generating single-frequency electrical signals. Frequency synthesizers are usually based upon phase-lock loop techniques in either analogue or digital domains for precise definition of the desired output frequency.