back door

from the GNU version of the Collaborative International Dictionary of English.

• A door in the back part of a building; hence, an indirect way.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A subsidiary entrance to a building or house at its rear, normally away from the street.
• noun A means of access, often secret and unprotected, to something.
• noun computing A secret means of access to a program or system.
• noun slang The anus, generally used in reference to anal sex.
• adjective US, baseball The path of a pitch which starts outside and then slides over the plate.
• verb To attempt to accomplish by indirect means, especially when direct means are proscribed.
• verb surfing To enter a tube by accelerating from behind; to surf into an already formed hollow wave, in contrast to the normal method of slowing to allow a surfable wave to form.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a secret or underhand means of access (to a place or a position)
• noun an undocumented way to get access to a computer system or the data it contains
• noun an entrance at the rear of a building