The Collaborative International Dictionary of English v.0.48:

pressurize \pressurize\ v. t.
   To increase the pressure on (a gas or liquid); as,
   pressurized oxygen.

   Syn: supercharge, apply pressure to.
        [WordNet 1.5]

   2. To increase and maintain a pressure in (a compartment or
      container), above that of the ambient pressure; -- of
      airplanes, space suits, and diving bells, for example; as,
      the airplane cabin was pressurized for the comfort of the
      passengers; the sterile room was pressurized to prevent
      contamination.
      [WordNet 1.5 + PJC]

   negatively pressurized maintained at a pressure somewhat
      below that of ambient pressure; -- a technique sometimes
      used to keep airborn materials in a room from escaping
      out; as, the biohazard lab was negatively pressurized to
      keep hazardous biological agents from escaping into the
      environment.
      [PJC]