Source : Webster's Revised Unabridged Dictionary (1913)

Back door \Back" door"\
   A door in the back part of a building; hence, an indirect
   way. --Atterbury.

Source : WordNet®

back door
     n 1: a secret or underhand means of access (to a place or a
          position); "he got his job through the back door"
     2: an entrance at the rear of a building [syn: {back entrance}]

Source : Free On-Line Dictionary of Computing

back door
     
         (Or "{trap door}", "{wormhole}").  A hole in the
        security of a system deliberately left in place by designers
        or maintainers.  The motivation for such holes is not always
        sinister; some {operating system}s, for example, come out of
        the box with privileged accounts intended for use by field
        service technicians or the vendor's maintenance programmers.
        See also {iron box}, {cracker}, {worm}, {logic bomb}.
     
        Historically, back doors have often lurked in systems longer
        than anyone expected or planned, and a few have become widely
        known.  The infamous {RTM} worm of late 1988, for example,
        used a back door in the {BSD} Unix "sendmail(8)" utility.
     
        {Ken Thompson}'s 1983 Turing Award lecture to the {ACM}
        revealed the existence of a back door in early {Unix} versions
        that may have qualified as the most fiendishly clever security
        hack of all time.  The C compiler contained code that would
        recognise when the "login" command was being recompiled and
        insert some code recognizing a password chosen by Thompson,
        giving him entry to the system whether or not an account had
        been created for him.
     
        Normally such a back door could be removed by removing it from
        the source code for the compiler and recompiling the compiler.
        But to recompile the compiler, you have to *use* the compiler
        - so Thompson also arranged that the compiler would *recognise
        when it was compiling a version of itself*, and insert into
        the recompiled compiler the code to insert into the recompiled
        "login" the code to allow Thompson entry - and, of course, the
        code to recognise itself and do the whole thing again the next
        time around!  And having done this once, he was then able to
        recompile the compiler from the original sources; the hack
        perpetuated itself invisibly, leaving the back door in place
        and active but with no trace in the sources.
     
        The talk that revealed this truly moby hack was published as
        ["Reflections on Trusting Trust", "Communications of the ACM
        27", 8 (August 1984), pp. 761--763].
     
        [{Jargon File}]
     
        (1995-04-25)