The Collaborative International Dictionary of English v.0.48:

Pressor \Press"or\, a. (Physiol.)
   Causing, or giving rise to, pressure or to an increase of
   pressure; as, pressor nerve fibers, stimulation of which
   excites the vasomotor center, thus causing a stronger
   contraction of the arteries and consequently an increase of
   the arterial blood pressure; -- opposed to depressor.
   --Landois & Stirling.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pressor
    adj 1: increasing (or tending to increase) blood pressure;
           "pressor reflexes"
    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]