n. One who or that which moderates, restrains, or represses.n. In microscopy, a device used to diminish the intensity or vary the character of the light which illuminates the object: it consists commonly of a screen of opal glass, ground glass, or glass of a pale-blue or neutral tint.n. An umpire; a judge.n. The person who presides at a meeting or disputation: now used chiefly in churches of the Presbyterian and Congregational order (as, the moderator of a presbytery or of the General Assembly), and in town-meetings in the United States.n. In the universities of Cambridge and Oxford, one of the public officers appointed to superintend the examinations for honors and degrees: so called because they formerly had to moderate or preside in the exercises of undergraduates for the degree of bachelor of arts.n. A moderator-lamp.n. In astrology, any one of the four principal points in a nativity upon which the native's fortunes are supposed to depend: the sun, moon, ascendant, or mid-heaven.n. In mech., a device for moderating or reducing the speed of an internal-combustion engine by the operation of a throttle-valve. Its action is the reverse of that of the accelerator.