(1):

(n.) One who, or that which, multiplies or increases number.

(2):

(n.) The number by which another number is multiplied. See the Note under Multiplication.

(3):

(n.) An instrument for multiplying or increasing by repetition or accumulation the intensity of a force or action, as heat or electricity. It is particularly used to render such a force or action appreciable or measurable when feeble. See Thermomultiplier.