Decompression was a decrease in air pressure.
When Jonathan Archer chose to interrogate the Osaarian pirate Orgoth for information about the Xindi in 2153, he placed the Osaarian into an airlock and began decompression it at a rate of 0.5 atmospheres per minute until his captive would start talking. He warned Orgoth that he better start soon or "in about forty seconds, you won't be able to." (ENT: "Anomaly (ENT)")