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]