The Collaborative International Dictionary of English v.0.48:

Janus \Ja"nus\, n. [L. See January.] (Rom. Antiq.)
   A Latin deity represented with two faces looking in opposite
   directions. Numa is said to have dedicated to Janus the
   covered passage at Rome, near the Forum, which is usually
   called the Temple of Janus. This passage was open in war and
   closed in peace. --Dr. W. Smith.
   [1913 Webster]

   Janus cloth, a fabric having both sides dressed, the sides
      being of different colors, -- used for reversible
      garments.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Janus
    n 1: (Roman mythology) the Roman god of doorways and passages;
         is depicted with two faces on opposite sides of his head

Moby Thesaurus II by Grady Ward, 1.0:

22 Moby Thesaurus words for "Janus":
   ambiguity, ambivalence, biformity, bifurcation, conjugation,
   dichotomy, doubleness, doublethink, doubling, dualism, duality,
   duplexity, duplication, duplicity, equivocality, halving, irony,
   pairing, polarity, twinning, two-facedness, twoness



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

Janus

   1. Distributed language with an ask/tell constraint system.

   qdjanus is a Janus-to-Prolog compiler for Sicstus Prolog
   and jc is compiles to C.

   ["Janus: A Step Towards Distributed Constraint Programming",
   V. Saraswat  et al in Logic
   Programming: Proc 1990 North Am Conf, S. Debray et al eds, MIT
   Press 1990].

   ["Programming in Janus", Saraswat, Kahn, and Levy].

   2. W.M. Waite, U Colorado.  Intermediate language, claimed as
   an implementation of UNCOL.  Used on CDC 6600.

   ["Experience with the Universal Intermediate Language Janus",
   B.K. Haddon et al, Soft Prac & Exp 8(5):601- 616 (Sep 1978)].