A fad or trend is any form of collective behavior that develops within a culture, a generation or social group in which a group of people enthusiastically follow an impulse for a short period.

Fads are objects or behaviors that achieve short-lived popularity but fade away. Fads are often seen as sudden, quick-spreading, and short-lived. Fads include diets, clothing, hairstyles, toys, and more. Some popular fads throughout history are toys such as yo-yos, hula hoops, and fad dances such as the Macarena, floss and the twist.

Similar to habits or customs but less durable, fads often result from an activity or behavior being perceived as emotionally popular or exciting within a peer group, or being deemed "cool" as often promoted by social networks. A fad is said to "catch on" when the number of people adopting it begins to increase to the point of being noteworthy. Fads often fade quickly when the perception of novelty is gone.