According to the Merriam-Webster Dictionary, fandom has two definitions.
1. :all the fans (as of a sport)
2. :the state or attitude of being a fan
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Unfortunately, these definitions do not accurately describe my understanding of "fandom".
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A better definition was submitted to Urban Dictionary, an online dictionary containing slang words and phrases.
1. A shortening of Fan Domain
2. A domain in the internet about a comic, a book, a tv show, a video game, etc. There are several fandoms all around the internet, where fans converge and the communities are really big. Many have forums, fanfics, fanart, etc.

My personal fandoms include: Doctor Who, Sherlock, and Supernatural. My favorite online community is Tumblr.

Here is "Tumblr Fandom Style", a parody of Gangnam Style.