In economics, a multiplier is a factor of proportionality that measures how much an endogenous variable changes in response to a change in some exogenous variable). For example, if variable x changes by k units, which causes another variable y to change by M × k units, then the multiplier is M. There are different types of multipliers, such as the spending multiplier and the tax multiplier, which are used to calculate the effect of a change in an economic factor on related variables in the economy. The multiplier effect refers to the proportional amount of increase or decrease in final income that results from an injection or withdrawal of spending.

In math, a multiplier is the number by which a multiplicand is multiplied to increase its value. For example, in the equation 4 x 5 = 20, 5 is the multiplier and 4 is the multiplicand. The product is the result of multiplying the multiplicand and the multiplier.