You will often see me (and any other person talking football) refer to the word gap, when speaking about both offense and defense, for example “the Mike backer has weakside A gap responsibility”. The specific definition of a gap in football terms would be a space between two offensive players or between an offensive player and the sideline.
A gap: Between the center and guard
B gap: Between the guard and tackle
C gap: Outside tackle
D gap: Outside tight end (if there is a tight end)
E gap: The outside shoulder of the outermost receiver (this is the edge)
From a defensive point of view, most defenses have what is called gap responsibility. Each defensive player will have a gap that he is assigned based on the playcall that he will have to fill against the run. As you can see, the gaps are A, B, C, D, and E. I have also added in the “alley” gaps which are essentially the space created by receiver and field spacing, where the “alley defender” will have to fill vs the run, and alleys will change based on offensive formation and alignment. They are usually brought into conversation when discussing specific run fits and not gaps, but I added them to visualize how every single gap between each offensive player is accounted for from a defensive perspective. There are 11 gaps created, 11 defensive players, and 11 gaps accounted for by those 11 defenders.
From an offensive point of view, gaps are usually used for the running game, specifically blocking schemes and ball-carrier direction. For example, a specific run concept may call for a downblock by the center if there is a defender in the backside A gap, but if the defender is head up with the backside guard or in the B gap, he would have a completely different assignment. From the ball carrier’s perspective, a run play may have a goal of being blocked so that he can run through a specific gap (gap/man), or it could be blocked so that the hole can open up in any gap (zone). A lead blocker will tend to have an aiming point of a specific gap as well.