noun
back gate; rear gate
Refers to a gate at the back or side of a property, often less formal than the main entrance.
学校の裏門から入ってください。
Please enter through the back gate of the school.
正門 is the main or front gate, while 裏門 is the back or rear gate.
Compound of 裏 (ura, 'back') and 門 (mon, 'gate').