The Collaborative International Dictionary of English v.0.48:

Immense \Im*mense"\, a. [L. immensus; pref. im- not + mensus, p.
   p. of metiri to measure: cf. F. immense. See Measure.]
   Immeasurable; unlimited. In commonest use: Very great; vast;
   huge. "Immense the power" --Pope. "Immense and boundless
   ocean." --Daniel.
   [1913 Webster]

         O Goodness infinite! Goodness immense!   --Milton.

   Syn: Infinite; immeasurable; illimitable; unbounded;
        unlimited; interminable; vast; prodigious; enormous;
        monstrous. See Enormous.
        [1913 Webster]

WordNet (r) 3.0 (2006):

immense
    adj 1: unusually great in size or amount or degree or especially
           extent or scope; "huge government spending"; "huge
           country estates"; "huge popular demand for higher
           education"; "a huge wave"; "the Los Angeles aqueduct
           winds like an immense snake along the base of the
           mountains"; "immense numbers of birds"; "at vast (or
           immense) expense"; "the vast reaches of outer space";
           "the vast accumulation of knowledge...which we call
           civilization"- W.R.Inge [syn: huge, immense, vast,
           Brobdingnagian]

Moby Thesaurus II by Grady Ward, 1.0:

110 Moby Thesaurus words for "immense":
   Atlantean, Brobdingnagian, Cyclopean, Gargantuan, Herculean,
   Homeric, abysmal, all-comprehensive, all-inclusive, amplitudinous,
   astronomic, astronomical, awesome, boundless, bulky, colossal,
   cosmic, countless, divine, elephantine, endless, enormous, epic,
   eternal, exhaustless, exquisite, extending everywhere, extensive,
   galactic, giant, giantlike, gigantic, gilt-edged, glorious, golden,
   heavenly, heroic, huge, humongous, illimitable, illimited,
   immeasurable, incalculable, incomprehensible, inexhaustible,
   infinite, infinitely continuous, innumerable, interminable,
   interminate, jumbo, king-size, large, limitless, magnificent,
   mammoth, marvelous, massive, massy, measureless, mighty, monster,
   monstrous, monumental, mountainous, no end of, outsize, overgrown,
   perpetual, prodigious, profound, sensational, shoreless, sizable,
   spacious, splendid, splendiferous, staggering, sterling,
   stupendous, sumless, super, superb, supereminent, superexcellent,
   superfine, termless, terrific, titanic, towering, tremendous,
   unbounded, uncircumscribed, unfathomable, universal, unlimited,
   unmeasurable, unmeasured, unnumbered, unplumbed, untold, vast,
   voluminous, weighty, without bound, without end, without limit,
   without measure, without number, wonderful