Mercurial describes something or someone that changes quickly and unpredictably, especially moods or opinions. A mercurial person might be laughing and cheerful one moment, then irritable and gloomy the next, with no obvious reason for the shift.

The word comes from Mercury, the Roman messenger god known for his speed and constant movement. (Mercury is also the name of the planet closest to the sun, which zips around faster than any other planet.) Just as Mercury darted quickly from place to place, a mercurial personality darts from one mood or idea to another.

You might have a friend with a mercurial sense of humor who finds something hilarious one day but doesn't think it's funny the next. A mercurial teacher might be strict on Monday and relaxed on Tuesday. Weather can be mercurial too, shifting from sunny to stormy without warning.

Being mercurial isn't necessarily bad; it just means unpredictable. Some creative people have mercurial minds that jump quickly between ideas, which can lead to innovation. But mercurial behavior can also frustrate others who prefer consistency and find it hard to keep up with rapid changes.