With Rex Ryan taking charge of the New York Jets and hiring former Buddy Ryan/Bears' safety Doug Plank, the term 46 defense has been tossed around lately.  Football fans have a vague idea of what the "46 Defense" is.  They know it involves an extra defender near the line of scrimmage to help stop the run.  But where does everyone line up -- is 46 just another way of saying "bring an 8th man into the box"?  Does it mean 4 down lineman and 6 linebackers?  Here's the complete explanation from Wikipedia, which does a really good job explaining the nuances of the scheme popularized by Buddy Ryan and the '85 Bears.  Note the 46 name does not represent the number of lineman & linebackers as is the case with 3-4, 4-3, and 5-2 defenses.  Instead, the 46 was named for the aforementioned safety Doug Plank, who was a vital component in the D's effectiveness.

Wikipedia Explains 46 Defense

"The "46" was an innovative defense with a unique defensive front; designed to confuse the quarterback. The line was shifted dramatically to the weak side (opposite the Tight End), with both Offensive Guards and the Center "covered" by the Left Defensive End and both Defensive Tackles. This front forced offenses to immediately account for the defenders directly lined up in front of them, making it considerably harder to execute blocking assignments such as pulling, trapping, and in general, pass protection. Moreover, the Right Defensive End would align outside of the Left Offensive Tackle, leaving him "on an island" when trying to block him.

Another key feature of the "46" is that both outside linebackers play on the same side of the formation. To avoid confusion the strong side linebacker (who is no longer lining up on the strong side) is frequently renamed to the Jack Linebacker. The linebackers line up behind the linemen two or three yards from the line of scrimmage. The primary tactic is to rush five to eight players on each play, either to get to the quarterback quickly or disrupt running plays.

The formation was very effective in the 1980s NFL because it often eliminated a team's running game and forced them to throw the ball. This was difficult for many teams at the time because most offensive passing games centered around the play action pass.

Currently, the "46" is rarely used in modern professional and college football because of the emergence of the West Coast Offense, popularized by San Francisco 49ers head coach Bill Walsh, and other offensive schemes that rely on short, timed passes from formations that use multiple receivers (though it should not be forgotten that the Chicago Bears won the NFC Championship for 1985 with their famed "46" right in the middle of Walsh's tenure with the 49ers). A major weakness of the "46" is that too many defensive players line up near the line of scrimmage to blitz, leaving areas wide open for receivers to catch passes. Also, the short, timed passes are thrown before the players blitzing have a chance to reach the quarterback. Another problem is that most teams do not have enough impact players to run the "46" as effectively as the 1980s Bears did. Those teams fielded some of the best front-seven defenses in the history of football, including such players as Mike Singletary, Richard Dent, Dan Hampton, and Wilber Marshall.

In Super Bowl XX, the Bears coincidentally scored 46 points in their victory against the New England Patriots.

The ideas of the "46" defense are more often used in today's game by bringing a fourth defensive back (usually the strong safety) up closer to the line of scrimmage (an eighth man in "the box" to help stop the run). This is done without the radical shift in formation seen in the "46" defense. Defenses today may also run safety blitzes and corner blitzes at crucial moments without committing wholly to the "46" defense."