The Collaborative International Dictionary of English v.0.48:

Imaginary \Im*ag"i*na*ry\, a. [L. imaginarius: cf. F.
   imaginaire.]
   Existing only in imagination or fancy; not real; fancied;
   visionary; ideal.
   [1913 Webster]

         Wilt thou add to all the griefs I suffer
         Imaginary ills and fancied tortures?     --Addison.
   [1913 Webster]

   Imaginary calculus See under Calculus.

   Imaginary expression or Imaginary quantity (Alg.), an
      algebraic expression which involves the impossible
      operation of taking the square root of a negative
      quantity; as, [root]-9, a + b [root]-1.

   Imaginary points, lines, surfaces, etc. (Geom.),
      points, lines, surfaces, etc., imagined to exist, although
      by reason of certain changes of a figure they have in fact
      ceased to have a real existence.

   Syn: Ideal; fanciful; chimerical; visionary; fancied; unreal;
        illusive.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Imaginary \Im*ag"i*na*ry\, n. (Alg.)
   An imaginary expression or quantity.
   [1913 Webster]

WordNet (r) 3.0 (2006):

imaginary
    adj 1: not based on fact; unreal; "the falsehood about some
           fanciful secret treaties"- F.D.Roosevelt; "a small
           child's imaginary friends"; "to create a notional world
           for oneself" [syn: fanciful, imaginary, notional]
    n 1: (mathematics) a number of the form a+bi where a and b are
         real numbers and i is the square root of -1 [syn: complex
         number, complex quantity, imaginary number,
         imaginary]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "imaginary":
   Barmecidal, Barmecide, abstract, air-built, airy, algorismic,
   algorithmic, aliquot, apparent, apparitional, autistic, cardinal,
   chimeric, chimerical, cloud-built, deceptive, decimal, delusional,
   delusionary, delusive, delusory, dereistic, differential, digital,
   dreamlike, dreamy, erroneous, ethereal, even, exponential,
   fallacious, false, fancied, fanciful, fantastic, fatuitous,
   fatuous, fictional, fictitious, fictive, figmental, figural,
   figurate, figurative, finite, fractional, gaseous, gossamery,
   hallucinatory, hypothetical, ideal, illusional, illusionary,
   illusive, illusory, imaginational, imaginative, imagined,
   immaterial, impair, impossible, infinite, integral, irrational,
   legendary, logarithmic, logometric, made-up, merely nominal,
   misleading, mythic, mythical, mythological, negative, nonexistent,
   not real, notional, numeral, numerary, numerative, numeric, odd,
   ordinal, ostensible, pair, phantasmagoric, phantasmal, phantasmic,
   phantom, phantomlike, positive, possible, prime, quixotic, radical,
   rarefied, rational, real, reciprocal, seeming, self-deceptive,
   self-deluding, shadowy, specious, spectral, spirituous,
   submultiple, subtile, subtle, supposititious, surd, tenuous,
   transcendental, unactual, unfounded, unreal, unrealistic,
   unsubstantial, untrue, vaporous, visional, visionary, windy