The term fanon refers to fan-made fiction. Fan fiction (also spelled fanfiction and often abbreviated fanfic) is an unlicensed story written by a fan that takes place in an established fictional universe, such as Star Wars. Such fiction is unrecognized by Lucasfilm and is therefore non-canon.
The term is a portmanteau of fan and canon.
Expanded Universe authors occasionally adopted fanon. One instance of this was the 501st Legion, a clone trooper unit that was named after a real charity organization. The fanon—or some elements of it, like the name in this case—thereby became part of Legends continuity.
The What's The Story? competition invited members of the Hyperspace fan club to submit fan-written backstories of selected characters, the best of which were accepted as their official backstories.
Expanded Universe authors occasionally adopted fanon. One instance of this was the 501st Legion, a clone trooper unit that was named after a real charity organization. The fanon—or some elements of it, like the name in this case—thereby became part of Legends continuity.
The What's The Story? competition invited members of the Hyperspace fan club to submit fan-written backstories of selected characters, the best of which were accepted as their official backstories.
Star Wars Fanon Wiki, the site for Star Wars fanon