The Collaborative International Dictionary of English v.0.48:

Vapor pressure \Vapor pressure\ or Vapor tension \Vapor tension\
   . (Physics)
   The pressure or tension of a confined body of vapor of a
   chemical substance in equilibrium with the solid or liquid
   form of the substance. The pressure of a given saturated
   vapor is a function of the temperature only, and may be
   measured by introducing a small quantity of the substance
   into a barometer and noting the depression of the column of
   mercury.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

vapor pressure
    n 1: the pressure exerted by a vapor; often understood to mean
         saturated vapor pressure (the vapor pressure of a vapor in
         contact with its liquid form) [syn: vapor pressure,
         vapour pressure]