Hypoxia was the medical condition when a person suffered from a lack of oxygen, and generally left them disoriented and gasping for air.
The administration of tri-ox compound helps a patient strengthen their cardiopulmonary system, and allows them to better withstand the effects hypoxia. James T. Kirk suffered from hypoxia during the koon-ut-kal-if-fee battle he fought with Spock in 2267. (TOS: "Amok Time"), (DS9: "The Sound of Her Voice")
When the USS Enterprise-D received a distress call from the USS Constantinople in 2365, and discovered that the ship had an outer-hull breach, it was feared that there may be an internal explosion, which would make the passengers aboard prone to severe hypoxia. (TNG: "The Schizoid Man")