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.
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.