Meaning of “back” - Learner’s Dictionary

back

noun [ C ] uk us /bæk/
NOT FRONT

A2 the part of something that is furthest from the front or in the opposite direction to the front:

He wrote his number down on the back of an envelope.
We were sitting near the back and couldn't see much.
I always keep a blanket in the back of the car.
BODY

A2 the part of your body from your shoulders to your bottom:

He was lying on his back.
SEAT

the part of a seat that you lean against when you are sitting:

the back of a chair
back to front (also US front to back)

with the back part of something where the front should be:

You've got your trousers on back to front.
in back of US

behind:

They sat in back of us on the plane.
→ See also a pat on the back , be (like) water off a duck's back , at/in the back of your mind , behind sb's back , be glad/happy, etc to see the back of sb/sth , be on sb's back , turn your back on sb/sth

(Definition of “back noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)