As usually used by synth enthusiasts, the term "discrete" refers to a circuit built out of individual transistors (or vacuum tubes), resistors, capacitors, etc., without the use of integrated circuits. Some believe that using discrete circuitry for audio circuits produces superior sonic results, while others dispute this. In general, a discrete circuit costs more, takes up more space, and uses more power than an integrated circuit that performs the same function.
However, sometimes discrete circuitry is the only option for replacing an integrated circuit that is out of production and no longer available. This has been a significant problem in both repairing older synths, and designing new ones, since about 1995. Many synths built between about 1978 and 1990 used synth-specific application ICs from Curtis or SSM; both companies are gone now and nearly all of the ICs that they produced are NLA. Another problem has been the disappearance of many of the operational transconductance amplifier circuits, which nearly all voltage controlled amplifier designs are based on, plus many voltage controlled filters.