A gas that has a critical temperature at or below 0ºC, where critical temperature is the temperature above which the gas cannot be liquefied by increasing the pressure
Ministry of Business, Innovation and Employment, Glossary of terms and acronyms, Wellington, New Zealand 2010. http://wwwsosh.govt.nz/resources/glossary-u.shtml, (Occupn Hlth and Sfty - Brit) {530}