The Collaborative International Dictionary of English v.0.48:

Premise \Prem"ise\, n.; pl. Premises. [Written also, less
   properly, premiss.] [F. pr['e]misse, fr. L. praemissus, p.
   p. of praemittere to send before; prae before + mittere to
   send. See Mission.]
   1. A proposition antecedently supposed or proved; something
      previously stated or assumed as the basis of further
      argument; a condition; a supposition.
      [1913 Webster]

            The premises observed,
            Thy will by my performance shall be served. --Shak.
      [1913 Webster]

   2. (Logic) Either of the first two propositions of a
      syllogism, from which the conclusion is drawn.
      [1913 Webster]

   Note: "All sinners deserve punishment: A B is a sinner."
         [1913 Webster] These propositions, which are the
         premises, being true or admitted, the conclusion
         follows, that A B deserves punishment.
         [1913 Webster]

               While the premises stand firm, it is impossible
               to shake the conclusion.           --Dr. H. More.
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   3. pl. (Law) Matters previously stated or set forth; esp.,
      that part in the beginning of a deed, the office of which
      is to express the grantor and grantee, and the land or
      thing granted or conveyed, and all that precedes the
      habendum; the thing demised or granted.
      [1913 Webster]

   4. pl. A piece of real estate; a building and its adjuncts;
      as, to lease premises; to trespass on another's premises.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Premiss \Prem"iss\, n.
   Premise. --Whately. I. Watts
   [1913 Webster]

WordNet (r) 3.0 (2006):

premiss
    n 1: a statement that is assumed to be true and from which a
         conclusion can be drawn; "on the assumption that he has
         been injured we can infer that he will not to play" [syn:
         premise, premiss, assumption]
    v 1: take something as preexisting and given [syn: premise,
         premiss]