Source : Webster's Revised Unabridged Dictionary (1913)

Vasoconstrictor \Vas`o*con*strict"or\, a. (Physiol.)
   Causing constriction of the blood vessels; as, the
   vasoconstrictor nerves, stimulation of which causes
   constriction of the blood vessels to which they go. These
   nerves are also called {vasohypertonic}.

Source : WordNet®

vasoconstrictor
     n : any agent that causes a narrowing of an opening of a blood
         vessel: cold or stress or nicotine or epinephrine or
         norepinephrine or angiotensin or vasopressin or certain
         drugs; maintains or increases blood pressure [syn: {vasoconstrictive}]