The Collaborative International Dictionary of English v.0.48:

Immortal \Im*mor"tal\, n.
   One who will never cease to be; one exempt from death, decay,
   or annihilation. --Bunyan.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Immortal \Im*mor"tal\, a. [L. immortalis; pref. im- not +
   mortalis mortal: cf. F. immortel. See Mortal, and cf.
   Immortelle.]
   1. Not mortal; exempt from liability to die; undying;
      imperishable; lasting forever; having unlimited, or
      eternal, existance.
      [1913 Webster]

            Unto the King eternal, immortal, invisible. --1 Tim.
                                                  i. 17.
      [1913 Webster]

            For my soul, what can it do to that,
            Being a thing immortal as itself?     --Shak.
      [1913 Webster]

   2. Connected with, or pertaining to immortality.
      [1913 Webster]

            I have immortal longings in me.       --Shak.
      [1913 Webster]

   3. Destined to live in all ages of this world; abiding;
      exempt from oblivion; imperishable; as, immortal fame.
      [1913 Webster]

            One of the few, immortal names,
            That were not born to die.            --Halleck.
      [1913 Webster]

   4. Great; excessive; grievous. [Obs.] --Hayward.
      [1913 Webster]

   Immortal flowers, immortelles; everlastings.

   Syn: Eternal; everlasting; never-ending; ceaseless;
        perpetual; continual; enduring; endless; imperishable;
        incorruptible; deathless; undying.
        [1913 Webster]

WordNet (r) 3.0 (2006):

immortal
    adj 1: not subject to death [ant: mortal]
    n 1: a person (such as an author) of enduring fame; "Shakespeare
         is one of the immortals"
    2: any supernatural being worshipped as controlling some part of
       the world or some aspect of life or who is the
       personification of a force [syn: deity, divinity, god,
       immortal]

Moby Thesaurus II by Grady Ward, 1.0:

193 Moby Thesaurus words for "immortal":
   Olympian, aggrandized, all-knowing, all-powerful, all-seeing,
   all-wise, almighty, amaranthine, apotheosized, awesome, beatified,
   beyond compare, beyond comparison, big, big name, boundless,
   canonized, ceaseless, celebrated, celebrity, changeless, classic,
   constant, constellation, creating, creative, cynosure, dea,
   deathless, deified, deity, deus, deva, devi, devil-god, divine,
   divinity, easily first, elevated, embalmed, eminent, endless,
   enduring, ennobled, enshrined, enthroned, eternal,
   eternally the same, ever-fresh, everlasting, exalted, excellent,
   facile princeps, fadeless, false god, famous, figure, folk hero,
   frozen, galaxy, genius, glorified, glorious, god, goddess, godlike,
   good, grand, great, great man, hallowed, heathen god, heavenly,
   held in awe, hero, heroine, high, high and mighty, highest, holy,
   honored, idol, immortalized, immutable, imperishable,
   important person, impregnable, incomparable, incorruptible,
   indelible, indestructible, ineffaceable, ineradicable,
   inexpugnable, inextinguishable, infinite, inimitable, invincible,
   invulnerable, just, lauded, legend, limitless, lion, lofty, loving,
   luminaries, luminary, luminous, magnified, majestic, making,
   man of mark, master spirit, matchless, merciful, mighty, name,
   never-dying, never-ending, nonperishable, notability, notable,
   nulli secundus, numinous, omnipotent, omnipresent, omniscient, one,
   pagan deity, peerless, permanent, perpetual, person of note,
   personage, pleiad, pop hero, popular hero, popular idol, praised,
   public figure, quenchless, radiant, remembered, renowned, sacred,
   sainted, sanctified, second to none, sempiternal, shaping, shrined,
   social lion, somebody, sovereign, star, sublime, sui generis,
   supereminent, superstar, supreme, the shining ones, throned,
   timeless, ubiquitous, unapproachable, unapproached, unbeatable,
   unbounded, unchanging, undampable, undefined, undestroyable,
   undying, unending, unequaled, unexampled, unexcelled, unfading,
   unique, unlimited, unmatchable, unmatched, unparagoned,
   unparalleled, unpeered, unquenchable, unrivaled, unsurpassable,
   unsurpassed, without equal, without parallel, worthy