A style of defense that features a basic alignment of three defensive linemen, four linebackers, and four defensive backs. Bud Wilkenson invented this defensive formation while coaching at the University of Oklahoma in the late 1940s.
The defensive line is comprised of two defensive ends and a nose tackle, then the four linebackers are utilized in many different ways and are often heavily involved in the pass rush. The defensive backs are comprised of two cornerbacks and two safeties.
The 3-4 defense relies on the three defensive linemen to be larger and stronger against the run, and requires a set of linebackers that are strong in many different facets of the game.
Since there are only three down linemen, the pass rush requires a linebacker to rush on almost every play. The 3-4 defense tends to rely on a variety of different looks, since the pass rush can come from any one of the four linebackers. The defensive backs are mostly responsible for covering players down-field on pass patterns.
A drawback of the 3-4 is the emphasis on linebackers needing to drop into pass coverage against (usually) quicker receivers and running backs. In general terms, the 3-4 usually struggles against a strong passing attack.
Hall of Fame coach Bill Parcels and New England Patriots coach Bill Belichick are widely known to use the 3-4 defense.