The Collaborative International Dictionary of English v.0.48:

Inessential \In`es*sen"tial\, a. [Pref. in- not + essential: cf.
   F. inessentiel.]
   [1913 Webster]
   1. Having no essence or being. --H. Brooke.
      [1913 Webster]

            The womb of inessential Naught.       --Shelley.
      [1913 Webster]

   2. Not essential; not required to achieve a given purpose;
      unessential; unnecessary.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

inessential
    adj 1: not basic or fundamental [syn: inessential,
           unessential] [ant: essential]
    2: not absolutely necessary
    n 1: anything that is not essential; "they discarded all their
         inessentials" [syn: inessential, nonessential] [ant:
         essential, necessary, necessity, requirement,
         requisite]