Fandom (consisting of fan plus the suffix -dom, as in kingdom, freedom, etc.) is a term used to refer to a subculture composed of fans characterized by a feeling of sympathy and camaraderie with others who share a common interest. Fans typically are interested in even minor details of the object(s) of their fandom and spend a significant portion of their time and energy involved with their interest, often as a part of a social network with particular practices (a fandom); this is what differentiates "fannish" (fandom-affiliated) fans from those with only a casual interest.
A fandom can grow up centered around any area of human interest or activity. The subject of fan interest can be narrowly defined, focused on something like an individual celebrity, or more widely defined, encompassing entire hobbies, genres or fashions. While it is now used to apply to groups of people fascinated with any subject, the term has its roots in those with an enthusiastic appreciation for sports. Merriam-Webster's dictionary traces the usage of the term back as far as 1903.
Fandom as a term can also be used in a broad sense to refer to the interconnected social networks of individual fandoms, many of which overlap.
Read more about Fandom: Organized Subculture, Fan Activities, In Film, Relationship With Industry