One of my all-time favorite football writers just put out a glossary of football terms!

Nice clean definition of Blitz:

A tactic where the defense rushes the quarterback with five or more defenders. At one time there were different words for different sorts of blitzes — “red dog” meant the linebackers rushed up the middle, while “blitz” usually referred to a rush from a defensive back — but now blitz is the catch-all term. “Blitz” is limited to where a defense rushes five or more defenders to differentiate it from a “stunt,” where a defense only rushes four or fewer but adjusts where those players will come from and at which angles.