The delay between a control being turned or a key being pressed and hearing the result in the output. Hardware synths have almost zero latency - but many software synths have a lag....
OB-X
"Oberheim released the polyphonic and programmable OB-X in 1979. Available in four-,
six-, and eight-voice configurations, it offered 32 program memories, polyphonic
portamento, and polyphonic sample-and-hold."