‘bottom’ and ‘behind’
Your bottom is the part of your body that you sit on. You can use bottom in conversation and in most kinds of writing.
If she could change any part of her body, it would be her bottom.
Speakers of American English usually say behind rather than ‘bottom’.
My behind ached from cycling all day.
In formal writing, you refer to this part of your body as your buttocks.
He strained the muscles on his shoulders and buttocks.
‘bum’ and ‘butt’
In conversation, some British speakers say bum instead of ‘bottom’, and some American speakers say butt. It is best to avoid both these words as many people think they are impolite.