The Collaborative International Dictionary of English v.0.48:

Unessential \Un`es*sen"tial\, a.
   1. Not essential; not of prime importance; not indispensable;
      unimportant. --Addison.
      [1913 Webster]

   2. Void of essence, or real being. [R.] --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Unessential \Un`es*sen"tial\, n.
   Something not constituting essence, or something which is not
   of absolute necessity; as, forms are among the unessentials
   of religion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

unessential
    adj 1: not basic or fundamental [syn: inessential,
           unessential] [ant: essential]