The Collaborative International Dictionary of English v.0.48:

Back door \Back" door"\
   A door in the back part of a building; hence, an indirect
   way. --Atterbury.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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" [syn:
         back door, backdoor]
    2: an undocumented way to get access to a computer system or the
       data it contains [syn: back door, backdoor]
    3: an entrance at the rear of a building [syn: back door,
       backdoor, back entrance]

Moby Thesaurus II by Grady Ward, 1.0:

105 Moby Thesaurus words for "back door":
   French door, afterpart, afterpiece, archway, back, back road,
   back seat, back side, back stairs, back street, back way,
   backstairs, barway, behind, bolt-hole, breech, bulkhead, by-lane,
   bypass, bypath, byroad, bystreet, byway, carriage entrance,
   cellar door, cellarway, clandestine, covert, covert way, detour,
   door, doorjamb, doorpost, doorway, escalier derobe, escape hatch,
   escape route, feline, front door, furtive, gate, gatepost, gateway,
   hatch, hatchway, heel, hidlings, hind end, hind part, hindhead,
   hole-and-corner, hugger-mugger, lintel, occiput, porch, portal,
   porte cochere, posterior, postern, privy, propylaeum, pylon, quiet,
   rear, rear end, rearward, reverse, roundabout way, scuttle,
   secret exit, secret passage, secret staircase, shifty, side door,
   side road, side street, skulking, slinking, slinky, sly, sneaking,
   sneaky, stealthy, stern, stile, storm door, surreptitious, tail,
   tail end, tailpiece, threshold, tollgate, trap, trap door,
   turnpike, turnstile, under-the-counter, under-the-table,
   undercover, underground, underground railroad, underground route,
   underhand, underhanded, unobtrusive



The Jargon File (version 4.4.7, 29 Dec 2003):

back door
 n.

    [common] 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 systems, for example, come out of the box with
    privileged accounts intended for use by field service technicians or the
    vendor's maintenance programmers. Syn. trap door; may also be called a
    wormhole. 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. Ken Thompson's
    1983 Turing Award lecture to the ACM admitted the existence of a back door
    in early Unix versions that may have qualified as the most fiendishly
    clever security hack of all time. In this scheme, the C compiler contained
    code that would recognize 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 recognize 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
    recognize 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 Turing lecture that reported this truly moby hack was later published
    as ?Reflections on Trusting Trust?, Communications of the ACM 27, 8 (August
    1984), pp. 761--763 (text available at http://www.acm.org/classics/). Ken
    Thompson has since confirmed that this hack was implemented and that the
    Trojan Horse code did appear in the login binary of a Unix Support group
    machine. Ken says the crocked compiler was never distributed. Your editor
    has heard two separate reports that suggest that the crocked login did make
    it out of Bell Labs, notably to BBN, and that it enabled at least one
    late-night login across the network by someone using the login name ?kt?.


The Free On-line Dictionary of Computing (30 December 2018):

back door
wormhole

    (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 systems, 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)