The Collaborative International Dictionary of English v.0.48:

Imagine \Im*ag"ine\, v. t. [imp. & p. p. Imagined; p. pr. &
   vb. n. Imagining.] [F. imaginer, L. imaginari, p. p.
   imaginatus, fr. imago image. See Image.]
   1. To form in the mind a notion or idea of; to form a mental
      image of; to conceive; to produce by the imagination.
      [1913 Webster]

            In the night, imagining some fear,
            How easy is a bush supposed a bear!   --Shak.
      [1913 Webster]

   2. To contrive in purpose; to scheme; to devise; to compass;
      to purpose. See Compass, v. t., 5.
      [1913 Webster]

            How long will ye imagine mischief against a man?
                                                  --Ps. lxii. 3.
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   3. To represent to one's self; to think; to believe. --Shak.

   Syn: To fancy; conceive; apprehend; think; believe; suppose;
        opine; deem; plan; scheme; devise.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Imagine \Im*ag"ine\, v. i.
   1. To form images or conceptions; to conceive; to devise.
      [1913 Webster]

   2. To think; to suppose.
      [1913 Webster]

            My sister is not so defenseless left
            As you imagine.                       --Milton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

imagine
    v 1: form a mental image of something that is not present or
         that is not the case; "Can you conceive of him as the
         president?" [syn: imagine, conceive of, ideate,
         envisage]
    2: expect, believe, or suppose; "I imagine she earned a lot of
       money with her new novel"; "I thought to find her in a bad
       state"; "he didn't think to find her in the kitchen"; "I
       guess she is angry at me for standing her up" [syn: think,
       opine, suppose, imagine, reckon, guess]

Moby Thesaurus II by Grady Ward, 1.0:

92 Moby Thesaurus words for "imagine":
   account as, assume, be afraid, believe, coin, conceit, conceive,
   conceptualize, conclude, concoct, conjecture, consider,
   contemplate, cook up, create, daresay, deduce, deem, devise,
   divine, dream, dream up, envisage, envision, esteem, estimate,
   expect, experience imaginatively, fabricate, fancy, fantasize,
   feature, feel, fictionalize, gather, grant, guess, hatch,
   have a hunch, have an idea, have an impression, have an inkling,
   have the idea, hold, hold as, ideate, image, infer, invent, judge,
   let, let be, look upon, look upon as, maintain, mold, opine,
   originate, picture, ponder, prefigure, presume, presuppose,
   presurmise, produce, provisionally accept, realize, reckon, regard,
   repute, say, see, set down as, shape, suppose, surmise, suspect,
   take, take for, take for granted, take it, take to be, think,
   think of, think up, trow, understand, view, view as, vision,
   visualize, ween