Also the "title" character of the musical Wicked, based on Maguire's book Wicked: The Life and Times of the Wicked Witch of the West.
Used as an epithet for one whose actions are cold-hearted (as in the classic view of a wicked witch), or used sarcastically when one's actions are misguidedly noble or terroristic in nature. (In Maguire's book, Elphaba spent several years as part of a terrorist group fighting the injustices the Wizard caused for the people and Animals of Oz.)
Irate Urbandictionary Editor: This is ridiculous. How do all these crap definitions get through?! I'm going to freaking hack this site and delete all the crap ones.
Annoyed, but Slightly Calmer Editor: Whatever you say, Elphaba.
"The green one? Dude the green witch?"
"Oh that's just Elphaba"
"She is freaking hot!"