A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure.
A vacuum chamber is a device that can create a vacuum inside of an enclosed container. Doctor Mora Pol subjected Odo to a vacuum chamber during his early experiments on the Changeling during the Occupation of Bajor. (DS9: "The Begotten")
In 2364, Captain Picard ordered that the Medical Lab on board the USS Enterprise-D be turned into a vacuum to contain the microbrain lifeform that was located there. (TNG: "Home Soil")
In 2370, with the discovery of a piece of wreckage from the USS Pegasus (lost twelve years previously), Commander Riker noted that the vacuum of space had preserved everything, including the dead bodies of the crew. (TNG: "The Pegasus")
Lt. Commander Data was able to function for extended periods of time in a vacuum. (TNG: "Birthright, Part I")