A fan, sometimes also called aficionado or supporter, is a person who is enthusiastically devoted to something, such as a band, a sports team or entertainer. Collectively, fans of a particular thing or person constitute its fanbase or fandom. They may show their enthusiasm by being members of a fan club, holding or participating in fan conventions, creating fanzines, writing fan mail, or by promoting the object of their interest and attention.

Fans usually have a strong enough interest that some changes in their lifestyles are made to accommodate devotion to the focal object. Fans have a desire for external involvement – they are motivated to demonstrate their involvement with the area of interest through certain behaviors (attending conventions, posting online, displaying team banners outside their homes, etc.). Fans often have a "wish to acquire" material objects related to the area of interest, such as a baseball hit by a famous slugger or a used guitar pick from their musical hero. As well, some fans have a desire for social interaction with other fans. This again may take many forms, from casual conversation, e-mail, chat rooms, and electronic mailing lists to regular face-to-face meetings such as fan club meetings and organized conventions.

There are several groups of fans that can be differentiated by the intensity level of their level of involvement or interest in the hobby (level of fanaticism) The likelihood for a subject of interest to be elevated to the level of fandom appears to be dictated by its complexity. Complexity allows further involvement of fans for a longer period of time because of the time needed to work the subject of interest 'out.' It also contributes to a greater sense of belonging because of the mental effort invested in the subject.

These are the things that make a fan a fan. One little mistake is not going to change the definition of what a fan is.