NOUN (1)

1. an unavoidable event;
- Example: "don't argue with the inevitable"


ADJECTIVE (2)

1. incapable of being avoided or prevented;
- Example: "the inevitable result"

2. invariably occurring or appearing;
- Example: "the inevitable changes of the seasons"

The Collaborative International Dictionary of English v.0.48:

Inevitable \In*ev"i*ta*ble\, a. [L. inevitabilis: cf. F.
   in['e]vitable. See In- not, and Evitable.]
   [1913 Webster]
   1. Not evitable; incapable of being shunned; unavoidable;
      certain. "The inevitable hour." --Gray.
      [1913 Webster]

            It was inevitable; it was necessary; it was planted
            in the nature of things.              --Burke.
      [1913 Webster]

   2. Irresistible. "Inevitable charms." --Dryden.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inevitable
    adj 1: incapable of being avoided or prevented; "the inevitable
           result" [ant: avertable, avertible, avoidable,
           evitable]
    2: invariably occurring or appearing; "the inevitable changes of
       the seasons"
    n 1: an unavoidable event; "don't argue with the inevitable"