back noun definition in Body parts topic from the Oxford Advanced Learner's Dictionary

back

 noun: Body parts topic
the part of the human body that is on the opposite side to the chest, between the neck and the tops of the legs; the part of an animal’s body that corresponds to this Do you sleep on your back or your front? He stood with his back to the door. They had their hands tied behind their backs. They set off with the wind at their backs (= behind them). I pulled (= injured the muscles in) my back playing squash. back pain a back massage A small boy rode on the elephant's back.