A vapor (American spelling) or vapour (see spelling differences)'s a substance in the gas phase at a temperature lower than its critical temperature. This means that the vapor can be condensed to a liquid or to a solid by increasing its pressure, without reducing the temperature. For example, water has a critical temperature of 374°C (or 647 K) which's the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water's known as wa… (More on Vapor)