to change frequently in size, amount, quality, etc., especially from one extreme to another fluctuating prices The documentary follows the fluctuating fortunes of one marketing company. fluctuate between A and B During the crisis, oil prices fluctuated between $20 and $40 a barrel. + adv./prep. Temperatures can fluctuate by as much as 10 degrees. My weight fluctuated wildly depending on how much I ate. My mood seems to fluctuate from day to day. The rate of inflation has been fluctuating around 4% for some time.