What Makes a Fandom?

Recently, I've been thinking of what makes a fandom a fandom.  Just last week, a story broke about a girl being harassed by fans of Steven Universe, and things have gone to pot afterwards, with people proclaiming that show as having "the worst fandom." (Out of respect for you dear readers, I link the story here, and I will be referencing it, but it is not a story for the weak of heart. Tread carefully.)