Definition of back line:
(n) :
(soccer) Collectively, the defenders of a team.
(n) :
(rugby) The players known as the backs, whose primary responsibility is defense.
(n) :
(field hockey) The line running across the goal to the corners of the playing field.
(n) :
(rugby) The 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).