What term describes trends that rise and fall quickly in popularity?

The term that describes trends which rise and fall quickly in popularity is "fad." Fads are characterized by their short-lived nature, often gaining rapid traction and widespread acceptance before disappearing just as quickly. They can be seen in various areas such as fashion, toys, or technology, where an item or idea suddenly becomes extremely popular for a brief period before interest wanes.

In contrast, a classic is a style that remains popular over a longer timeframe, showing enduring appeal instead of fleeting popularity. A trend indicates a general direction in which something is developing or changing, but it typically lasts longer than a fad and may evolve into a more established style or norm. Style refers more broadly to the manner of expression, encompassing various trends and fads but not limited to their quick rise and fall. Thus, "fad" is the most accurate term for quickly fluctuating trends.