Backside (noun)

1. The rear or posterior part of a person or animal.
2. The rear or hind part of an object or surface.
3. The part of a surface, object, or location that is furthest from the front.

Origin:

From the middle english bakside, from the old english bæcsīde, from bæc ("back") + sīde ("side"). first recorded in the 14th century.

Examples:

1. The cat rubbed against my leg and then jumped up onto the sofa, showing me its backside.
2. The backside of the photo showed that it had been ripped in half and then taped back together.
3. I turned the painting over to examine the backside, where I found the artist's signature.
4. I saw the backside of the building, which had a fire escape and several air conditioning units.
5. The backside of the mountain was much steeper than the front and covered in dense forest.