means The degree to which something—prices, moods, chemicals—swings unpredictably and rapidly up and down over time.

from From the Latin 'volatilis,' meaning 'flying' or 'fleeting,' built on 'volare,' to fly. The word literally began with birds and winged things, then drifted to substances that 'fly off' into vapor (volatile chemicals that evaporate), and finally to anything that won't sit still—tempers, markets, situations that could take flight at any moment. The image holds throughout: something that refuses to stay grounded.