Behind and Back end

Behind noun - The part of the body upon which someone sits.

Back end and behind are semantically related In some cases you can use "Back end" instead a noun "Behind".

Back end noun - The side of an object that is opposite its front.

Behind and back end are semantically related in rear topic. Sometimes you can use "Behind" instead a noun phrase "Back end", if it concerns topics such as back or end part, animate rear end.