Fluctuate means to rise and fall irregularly, changing back and forth without following a predictable pattern. When something fluctuates, it goes up and down repeatedly, like a yo-yo that never quite settles in one place.

Temperatures fluctuate throughout the day, warming up in the afternoon and cooling down at night. Gas prices fluctuate from week to week, sometimes higher and sometimes lower. A student's mood might fluctuate during a long school day, feeling energetic in the morning, tired before lunch, and excited again during recess.

The word suggests movement that's somewhat unpredictable. If your basketball team's performance fluctuates, you might win three games in a row, then lose two, then win again. If your little brother's interest in his hobbies fluctuates, he might be obsessed with drawing one week and completely focused on building with LEGO bricks the next.

Scientists track how things fluctuate to find patterns. Stock market prices fluctuate constantly, which is why investors watch them so carefully. Even your energy levels probably fluctuate throughout the week: you might feel strong and focused on Tuesday but sluggish on Thursday, even though you're doing similar activities both days.