On a multitimbral synth, an alternate output, or set of outputs, that one section or patch can be routed to, for mixing or processing independently of the rest of the synth's mixed output. Direct outs usually bypass the synth's built-in effects, and generally a signal that is routed to a direct out disappears from the main outputs.
Direct outs were frequently found on multitimbral synths designed in the 1990s or early 2000s, when virtual mixing was a common mixdown and production technique; one synth would often be asked perform a number of parts simultaneously, all under MIDI control. With the advent of modern DAWs, there is less motivation to engage in virtual mixing, and so less need for multitimbrality and direct outs.