Mike Linebacker
The Mike is the middle linebacker — the defense's signal-caller and the player offenses identify on every snap.
The Mike linebacker is the middle linebacker. In a 4-3 he's the man between the two outside backers; in a 3-4 he's one of two inside linebackers, usually the one wearing the green dot in his helmet. He's the defense's signal-caller, the player who relays the call from the sideline and sets the front before the snap.
When a quarterback points and yells 'Mike, 51 is the Mike!' he's identifying the linebacker who anchors the protection scheme. The offensive line's BOB and slide rules are built around the Mike's location, and changing his alignment changes the protection. That's why defenses move the Mike pre-snap to disguise pressure — the protection adjusts on the fly, sometimes badly.
The Mike's job is the hardest in the front seven. He has to fit the run, drop in coverage, communicate the front, handle motion adjustments, and react to formation shifts — all in the seven seconds between the offense's break of the huddle and the snap. The best Mikes (Ray Lewis, Patrick Willis, Bobby Wagner) made everyone around them play faster.