The Collaborative International Dictionary of English v.0.48:

Illative \Il"la*tive\, a. [L. illativus: cf. F. illatif.]
   Relating to, dependent on, or denoting, illation;
   inferential; conclusive; as, an illative consequence or
   proposition; an illative word, as then, therefore, etc.
   [1913 Webster]

   Illative conversion (Logic), a converse or reverse
      statement of a proposition which in that form must be true
      because the original proposition is true.

   Illative sense (Metaph.), the faculty of the mind by which
      it apprehends the conditions and determines upon the
      correctness of inferences.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Illative \Il"la*tive\, n.
   An illative particle, as for, because.
   [1913 Webster]

WordNet (r) 3.0 (2006):

illative
    adj 1: relating to or having the nature of illation or
           inference; "the illative faculty of the mind" [syn:
           inferential, illative]
    2: resembling or dependent on or arrived at by inference; "an
       illative conclusion"; "inferential reasoning" [syn:
       illative, inferential]
    3: expressing or preceding an inference; "`therefore' is an
       illative word"