Plain English Meaning

An aircraft where the air inside the cabin is the same thinness as the air outside. As the aircraft climbs, the air inside gets thinner along with the air outside.

Full Definition

An aircraft whose cabin is not sealed and actively maintained at a higher air pressure than the surrounding atmosphere. The cabin pressure inside the aircraft is the same as the outside air pressure at whatever altitude the aircraft is flying.

Context Anchor

Seen in discussions of high-altitude flying, oxygen use, and pitot/static system problems where cabin air pressure may affect instrument indications.

Derivation

Pressurize' comes from the Latin 'pressura' (a pressing). The prefix 'un-' means 'not.' So an unpressurized aircraft is one that does not press extra air into the cabin to keep it denser than the outside air.