A vacuum chamber was an enclosure from which air and other gases were removed by a pump. The resulting low pressure, commonly referred to as a vacuum, allowed researchers to conduct physical experiments or to test mechanical devices in an environment like outer space.
Doctor Mora Pol's experiments on Odo at the Bajoran Center for Science during the Cardassian Occupation of Bajor included subjecting the Changeling to a vacuum chamber. Years later, Odo recalled being subjected to the experiments, which were ultimately inconclusive. (DS9: "The Begotten")