The Collaborative International Dictionary of English v.0.48:

Vasodilator \Vas`o*di*lat"or\, n. [L. vas a vessel + dilator.]
   (Medicine & Physiology)
   A substance which causes dilation of blood vessels.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Vasodilator \Vas`o*di*lat"or\, a. [L. vas a vessel + dilator.]
   (Physiol.)
   Causing dilation or relaxation of the blood vessels; as, the
   vasodilator nerves, stimulation of which causes dilation of
   the blood vessels to which they go. These nerves are also
   called vaso-inhibitory, and vasohypotonic nerves, since
   their stimulation causes relaxation and rest.
   [1913 Webster]

WordNet (r) 3.0 (2006):

vasodilator
    n 1: a drug that causes dilation of blood vessels [syn:
         vasodilator, vasodilative]