One who holds the timing of many so they sound as one. The conductor plays nothing and decides everything.
Latin conductor (one who leads together, a contractor), from conducere (to lead together, escort), from com- (together) + ducere (to lead); one who holds the timing of many so they move, or sound, as one.
One who holds the timing of many so they sound as one, playing no note but deciding all of them. Leading-together is the root and the whole job at once.