General term for a rear door of a building or vehicle; also used for the tailgate of a station wagon or SUV.
Please enter through the back door of the shop.
I opened the tailgate and loaded the luggage.
backdoor (computing)
In computing, a hidden method of bypassing normal authentication or encryption in a system.
The hacker installed a backdoor in the system.
backdoor pitch (baseball)
In baseball, a pitch that appears to be outside the strike zone but catches the edge of the plate, often used for a strike.
The pitcher struck the batter out with a backdoor slider.
From English 'back door'.