These verbs mean to move back and forth, up and down, or to and fro: Swing usually applies to arclike movement of something attached at one extremity and free at the other: The ship's lanterns swung violently in the raging storm.Oscillate literally refers to a steady back-and-forth motion, as that of a pendulum; figuratively, it denotes vacillation, as between conflicting purposes: “a king … oscillating between fear of Rome and desire of independence” (Walter Besant). Sway suggests the movement of something unsteady, light, or flexible: “thousands of the little yellow blossoms all swaying to the light wind” (W.H. Hudson). To rock is to swing gently or rhythmically or sway or tilt violently: “The ruins of the ancient church seemed actually to rock and threaten to fall” (Sir Walter Scott). Vibrate implies quick periodic oscillations; it can also suggest trembling, pulsating, or quivering: “Music, when soft voices die,/Vibrates in the memory” (Percy Bysshe Shelley). Fluctuate implies fairly constant alternating change: “Prices fluctuated violently from the irregularity of the crops” (Lesley B. Simpson). Undulate refers to smooth wavelike movement: “gleaming seaweed that curls and undulates with the tide” (Willa Cather). Waver suggests unsteady, uncertain movement: A police officer stopped the driver who was wavering from lane to lane.