Merriam-Webster Collegiate Dictionary - decrease

 I. verb  (~d; decreasing)  Etymology: Middle English decreessen, from Anglo-French decrestre, from Latin decrescere, from de- + crescere to grow — more at crescent  Date: 14th century  intransitive verb to grow progressively less (as in size, amount, number, or intensity)  transitive verb to cause to ~  • decreasingly adverb Synonyms:  ~, lessen, diminish, reduce, abate, dwindle mean to grow or make less. ~ suggests a progressive decline in size, amount, numbers, or intensity slowly ~d the amount of pressure. lessen suggests a decline in amount rather than in number has been unable to lessen her debt. diminish emphasizes a perceptible loss and implies its subtraction from a total his visual acuity has diminished. reduce implies a bringing down or lowering you must reduce your caloric intake. abate implies a reducing of something excessive or oppressive in force or amount the storm abated. dwindle implies progressive lessening and is applied to things growing visibly smaller their provisions dwindled slowly.  II. noun  Date: 14th century  1. the process of decreasing  2. an amount of diminution ; reduction