The Collaborative International Dictionary of English v.0.48:

Undefined \Undefined\
   See defined.

WordNet (r) 3.0 (2006):

undefined
    adj 1: not precisely limited, determined, or distinguished; "an
           undefined term"; "undefined authority"; "some undefined
           sense of excitement"; "vague feelings of sadness"; "a
           vague uneasiness" [syn: undefined, vague] [ant:
           defined]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "undefined":
   aleatoric, aleatory, all-knowing, all-powerful, all-seeing,
   all-wise, almighty, amorphic, amorphous, anarchic, anonymous,
   baggy, blear, bleared, bleary, blobby, blurred, blurry, boundless,
   broad, chance, chancy, changeless, chaotic, characterless,
   confused, creating, creative, cryptonymic, cryptonymous, dark, dim,
   disordered, disorderly, eternal, eternally the same, everlasting,
   faint, featureless, feeble, filmy, foggy, formless, fuzzy, general,
   glorious, good, half-seen, half-visible, hallowed, hazy, highest,
   hit-or-miss, holy, ill-defined, immortal, immutable, imprecise,
   inaccurate, inchoate, incognito, incoherent, inconspicuous,
   indecisive, indefinable, indefinite, indeterminable, indeterminate,
   indistinct, indistinguishable, inexact, infinite, inform,
   innominate, just, kaleidoscopic, lax, limitless, loose, loving,
   low-profile, luminous, lumpen, majestic, making, merciful,
   merely glimpsed, misty, nameless, nondescript, nonspecific,
   numinous, obscure, omnipotent, omnipresent, omniscient, one,
   orderless, out of focus, pale, permanent, perpetual, radiant,
   random, sacred, semivisible, shadowed forth, shadowy, shapeless,
   shaping, sovereign, stochastic, supreme, sweeping, timeless,
   ubiquitous, unacknowledged, unbounded, uncertain, unchanging,
   unclear, undesignated, undestined, undetermined, unidentified,
   unknown, unlimited, unnamed, unordered, unorganized, unplain,
   unrecognizable, unspecified, vague, veiled, weak, without a name



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

undefined

    The value of a variable that has not been set or a
   function that does not return anything.  In some programming
   languages, e.g. Perl, JavaScript, undefined is a named
   constant that can be used to explicitly set a variable or return
   undefined or can be passed as an actual argument.  Other
   languages, e.g. Java, call it "null", but note that the null
   in relational database programming is subtly different.

   Many languages provide a built-in function to test whether an
   expression is undefined, e.g. Perl's defined() function.

   Attempting to operate on an undefined value, e.g. add it to a
   number or append it to a string, may either raise an error or
   result in the undefined value being converted (cast) to some
   appropriate value, e.g. false, zero or empty string, according
   to the type of expression.

   This definition is an example of a paradox.

   (2012-12-02)