Decrease, lessen, diminish, reduce, abate, dwindle denote to make or grow less, but they are not freely interchangeable.
Decrease and lessen are often employed in place of any of the others.
Decrease normally retains, even in the transitive, an implication of the process of growing less, and suggests progressive decline.
Lessen is a close synonym of decrease but the latter is to be preferred in contexts employing specific numbers; thus, it is idiomatic to say that a fever has lessened or that it has decreased from 101° to 99°.
Diminish is a more precise word when the ideas of taking away or subtraction by an agent and of resultant perceptible loss are to be emphasized.
Reduce adds to diminish the implication of bringing down, or lowering; it suggests more than any of the others the operation of a personal agent.
Reduce also is applicable to lowering in rank, status, or condition.
Abate differs from diminish and reduce in its presupposition of something excessive in force, intensity, or amount and in its strong implication of moderation or, especially when referred to taxes or imposts, of deduction.
Dwindle, like decrease, implies progressive lessening, but is more often applied to things capable of growing visibly smaller. It specifically connotes approach to a vanishing point.