What does pressurized mean?

In a space that is "pressurized," the air pressure has been made higher than it would be naturally. This is accomplished by forcing (pumping) air into the space. A common application is in aircraft, where, to allow normal breathing, the cabin air pressure has to be higher than the "thin" (low-pressure) ambient air found at high altitudes. Conversely, a diving suit may have to be pressurized to withstand the water pressure found below the surface. (In fact, a space can be pressurized with any gas; but if not specified or understood from the context, it is generally assumed that the gas is air. Gases that are highly pressured into storage containers -- cylinders, tanks, and the like -- are said to be "compressed" or "liquified," rather than merely pressurized.) In a "negatively-pressurized" (or "negative-pressure") space, the air is forcibly exhausted, which leaves that space at a lower pressure than ambient. The purpose is usually to exhaust air that is contaminated (e.g., with pathogens or other hazardous materials or gases), and allow it to be replaced with cleaner surrounding air which enters the space because it is at normal pressure.