back line

plural back lines

1. ball gamesCollectively, the defenders of a team.

   Battling Bolton disputed the goal, claiming Drogba was offside, but the Ivorian appeared to have timed his angled run across the visitors back line to perfection to race onto a fine Michael Essien pass and ensure Chelsea moved back into fourth place in the Premier League.

   2010 December 29, Chris Whyatt, “Chelsea 1 - 0 Bolton”, in BBC

2. field hockeyThe line running across the goal to the corners of the playing field.
3. ball gamesThe players known as the backs, whose primary responsibility is defense.

   A running fullback radically improves the attacking prowess of any back line, particularly if she is able to read a play, knowing when it's time to hit the line at pace or to join the line to act as an extra attacker.

   2011, Mathew Brown, Patrick Guthrie, Greg Growden, Rugby For Dummies, →ISBN, page 58

4. ball gamesThe imaginary tactical line across the field that passes through the backs, through their primary line of attack/defence (i.e. excluding any players in a more backward defensive position).