Fetching details from the dictionary...

back
reverse; rear side; back (e.g. of a chair); spine (of a book)
height; stature
ridge (of a mountain)
猫が背を丸めて寝ている。
The cat is sleeping with its back curled up.
背に荷物を背負って山道を歩いた。
I walked the mountain path carrying luggage on my back.
椅子の背にもたれてゆっくり休んだ。
I leaned against the back of the chair and rested.