A defensive formation popularized by coach Buddy Ryan that involves 4 defensive lineman (2 defensive ends, 2 defensive tackles), 3 linebackers, and 4 defensive backs (2 cornerbacks and 2 safeties). The 46 defense is somewhat similar to the 4-3 defense. The linebackers are usually shifted towards the offense's weak side and is designed to put pressure on the quarterback.
Due to the popularity of the West Coast Offense (which involves quick, short passes) the 46 defense is no longer popular among NFL football teams. The exception is Buddy Ryan's sons Rex Ryan and Rob Ryan, who often still employ this formation in their defensive schemes.
The term "46" was coined from the jersey of Bears linebacker Doug Plank, who played for Buddy Ryan in the 1980s when he developed the defense.