In synthesizer lingo, this refers to the gradual sliding or slurring of output frequency, from one note to the next, as a melody is played. Found on most monophonic and a few polyphonic synths, portamento can be a spectacular effect when not over-used. Polyphonic portamento is problematic to implement, because it has to be done on a per-voice basis, but voice allocation can lead to surprising results.

On an analog synthesizer, the circuit used to produce portamento is called a lag processor.