What is fei?

Some articles on fei:

Syaoran (Tsubasa: Reservoir Chronicle) - Appearances
... Syaoran was created by the sorcerer Fei-Wang Reed in order to find Sakura's feathers no matter who he kills in the process ... Syaoran loses all his emotions and is used by Fei-Wang to collect feathers throughout dimensions on his own, betraying all of his friends ... is once again emotionless when the original Syaoran, Kurogane, and Fai go to fight against Fei-Wang ...