1. vacillate To fluctuate (L. fluctus, a wave) is to move like a wave with alternate rise and fall. A pendulum oscillates; waves fluctuate or undulate; a light or a flame wavers; a frightened steed s Synonyms: fluctuate, hesitate, swerve, vacillate, veer, oscillate, undulate, vary, waver Antonyms: abide, adhere, hold fast, persist, stand fast, stay, stick