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]