An avid fan is someone who is passionate and enthusiastic about a particular topic or activity. A NUT is an informal term for someone who is obsessed with something, a BUFF is someone who is knowledgeable and enthusiastic about a particular subject, and a GEEK is someone who is very knowledgeable and enthusiastic about a particular subject. Therefore, an avid fan can be described as a NUT, BUFF, or GEEK depending on the level of enthusiasm and knowledge they have for a particular topic or activity.