The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Vasoconstrictor \Vas`o*con*strict"or\, n. (Medicine, Physiology)
   A substance which causes constriction of the blood vessels.
   Such substances are used in medicine to raise abnormally low
   blood pressure.
   [PJC]

WordNet (r) 3.0 (2006):

vasoconstrictor
    n 1: 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:
         vasoconstrictor, vasoconstrictive, pressor]