A Linebacker is commonly referred to as an LB.
Yes, it is a football term
Linebackers are part of the defensive team, and line up approximately 3 to 5 yards (4 m) behind the line of scrimmage, behind the defensive linemen, and therefore "back up the line."
Uau, that info is very complex.
List of formations in American football
The following is a list of common and historically significant formations in American football. In football, the formation describes how the players in a team are positioned on the field. Many variations are possible on both sides of the ball, depending on the strategy being employed. On offense, the formation must include at least seven players on the line of scrimmage, including a center to start the play by snapping the ball. 
There are no restrictions on the arrangement of defensive players, and
Linebacker
A linebacker (LB or backer) is a playing position in American football and Canadian football. Linebackers are members of the defensive team, and line up approximately three to five yards (4 m) behind the line of scrimmage, behind the defensive linemen, and therefore "back up the line". Linebackers generally align themselves before the ball is snapped by standing upright in a "two-point stance" (as opposed to the defensive linemen, who put one or two hands on the ground for a "