Definitions:
the rear surface of the human body from the shoulders to the hips

the side or part of something that is away from the spectator or from the direction in which it moves or faces; the rear

a player in a team game who plays in a defensive position behind the forwards

the grounds of cambridge colleges which back on to the river cam.

in the opposite direction from the one that one is facing or travelling towards

so as to return to an earlier or normal position or condition

in or into the past

in return

give financial, material, or moral support to

walk or drive backwards

cover the back of (an article) in order to support, protect, or decorate it

lie behind or at the back of

of or at the back of something

from or relating to the past

directed towards the rear or in a reversed course

(of a sound) articulated at the back of the mouth