a door at the back or side of a building, or at the back of a vehicle

There's someone at the back door. 后门处有一个人。

He opened the back door and gestured for her to get into the car. 他打开后门，示意她上车。

used to refer to an indirect way of achieving something, especially so that people do not know about it and so cannot object to it

HMI is to be privatised by the back door because the half of HMI who will not be employed by it will have to go and set up as private consultants. 皇家教育监督局其实是以一种偷偷摸摸的方式私有化，因为那部分不再被雇用的人员将不得不自立门户成为私人顾问。

The data collection exercise was designed to open the back door to an energy consumption tax on home owners. 资料收集的设计方式其实是为将来向房屋所有人征收能源消费税偷偷铺路。