1 challenge, dare, face, confront, brave, stand up to, flout, brazen out, thumb one's nose at, Colloq Brit cock a snook at:
The defendant defied the prosecutor to prove the allegations.
2 frustrate, thwart, baffle, resist, withstand, repel, disobey, repulse:
Her feats of legerdemain defy the imagination. Copeley has invented a device that defies the law of gravity.