The word decrease has its origin in Latin and is taken from the word dēcrēscere; dē = de and crēscere means to grow. Later, it was adapted into Old French as decreistre and then into Middle English as decresen. When something decreases, it is said to grow lesser or smaller. The intensity, amount or number of the thing concerned reduces. The act or process of decreasing is called decrease and is used a noun. For example: there was a decrease in his temperature as the fever subsided. Another example: there was a sharp decrease in the sales.
The amount by which something decreases is also called 'decrease'. For example: there was a 12 percent decrease in sales. Twelve is the amount of decrease or the decrease. Abate, diminish, drain, dwindle, ebb, lessen, let up, peter, rebate, reduce, tail away, taper, abatement, curtailment, cut and cutback are some of the words that mean decrease.