Source : Webster's Revised Unabridged Dictionary (1913)

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

            The constructive fingers of Watts.    --Emerson.

   2. Derived from, or depending on, construction or
      interpretation; not directly expressed, but inferred.

   {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.

Source : WordNet®

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"

Source : Free On-Line Dictionary of Computing

constructive
     
         A proof that something exists is "constructive"
        if it provides a method for actually constructing it.
        {Cantor}'s proof that the {real number}s are {uncountable} can
        be thought of as a *non-constructive* proof that {irrational
        number}s exist.  (There are easy constructive proofs, too; but
        there are existence theorems with no known constructive
        proof).
     
        Obviously, all else being equal, constructive proofs are
        better than non-constructive proofs.  A few mathematicians
        actually reject *all* non-constructive arguments as invalid;
        this means, for instance, that the law of the {excluded
        middle} (either P or not-P must hold, whatever P is) has to
        go; this makes proof by contradiction invalid.  See
        {intuitionistic logic} for more information on this.
     
        Most mathematicians are perfectly happy with non-constructive
        proofs; however, the constructive approach is popular in
        theoretical computer science, both because computer scientists
        are less given to abstraction than mathematicians and because
        {intuitionistic logic} turns out to be the right theory for a
        theoretical treatment of the foundations of computer science.
     
        (1995-04-13)