The intensity of a played note. Technically, volume refers to the objective measurement of the note's amplitude, while the term loudness refers to the subjective perception, but synth players frequently use "volume" in either context. It is worth noting that volume can effect the perception of pitch; low frequencies sometimes sound lower at higher volumes.
A MIDIcontinuous controller, which instructs the synth to adjust its output volume level. Often called "main volume" or "master volume". It is assigned to controller #7.