oscillation (n.)

"kind of vibration in which a body swings backward and forward," 1650s, from French oscillation and directly from Latin oscillationem (nominative oscillatio), noun of action from past-participle stem of oscillare "to swing," from oscillum "a swing," which usually is identified with the oscillum that meant "little face" (literally "little mouth"), a mask of open-mouthed Bacchus hung up in vineyards as a charm (the sense evolution would be via the notion of "swinging in the breeze"); from PIE *os- "mouth" (see oral). Figurative use, in reference to a swinging back and forth (in opinion, attitude, etc.) is by 1798.