The Collaborative International Dictionary of English v.0.48:

Constructive \Con*struct"ive\, a. [Cf. F. constructif.]
   1. Having ability to construct or form; employed in
      construction; as, to exhibit constructive power.
      [1913 Webster]

            The constructive fingers of Watts.    --Emerson.
      [1913 Webster]

   2. Derived from, or depending on, construction, inference, or
      interpretation; not directly expressed, but inferred.
      [1913 Webster]

   3. helpful; promoting improvement; intended to help; as,
      constructive criticism; constructive suggestions.
      Contrasted with destructive.
      [PJC]

   Constructive crimes (Law), acts having effects analogous to
      those of some statutory or common law crimes; as,
      constructive treason. Constructive crimes are no longer
      recognized by the courts.

   Constructive notice, notice imputed by construction of law.


   Constructive trust, a trust which may be assumed to exist,
      though no actual mention of it be made.
      [1913 Webster]

WordNet (r) 3.0 (2006):

constructive
    adj 1: constructing or tending to construct or improve or
           promote development; "constructive criticism"; "a
           constructive attitude"; "a constructive philosophy";
           "constructive permission" [ant: destructive]
    2: emphasizing what is laudable or hopeful or to the good;
       "constructive criticism"