Fluctuating describes something that has unpredictable ups and downs. Fluctuating often refers to changing numbers or quantities of something, like fluctuating stock prices that go up and down.
The word fluctuating originated in the 1600s, coming from the Latin word fluctuatus, a form of fluctuare, which means "to undulate." If something is fluctuating, it's unstable — it can swing back and forth. When the weather is fluctuating that means it can change at any moment, dumping snow on your town one minute and then producing warm weather the next. Moods, demand, morals, prices — anything that has unpredictable shifts can be described as fluctuating.