Liquid air sounds like a contradiction in terms but in fact, it’s not. Air, when cooled enough (-318°F or -195°C), condenses into a liquid and when cooled further even freezes solid. We’re familiar with this phenomenon in the case of water: steam condenses to liquid water that freezes to ice. Or, to put it the other way, ice melts to form water at 0°C and boils to produce steam at 100°C. (These temperatures change as the pressure changes. At high altitudes, for example, water boils at a lower temperature because of the lower air pressure.)
Liquid air can become a safety concern for the low temperature and for its high
oxygen concentration. This short paper on Liquid Air Safety explains the concern about liquid air instability.