Real Dictionary

behind in American English
in or to the rear or back
adjective: following
adverb: at or toward the rear; rearward
noun: the buttocks
preposition: at or toward the rear of

behind in British English
adjective: in a position further back; retarded
adverb: in or to a position further back; following
noun: the buttocks
preposition: in or to a position further back than; at the rear of; at the back of

behind noun use
countable noun: Your behind is the part of your body that you sit on.

behind preposition and adverb uses
preposition: If something is behind a thing or person, it is on the other side of them from you, or nearer their back rather than their front.