The substance used in refrigeration to remove heat from an area and carry it away. It generally boils in atmosphere at a much lower temperature then water and has certain properties.
Different refrigerants are used depending on the application. Freezers generally use R22 whilst ordinary refrigerator cabinets use 134A, R12 or one of its replacements.
Historical refrigerants include ammonia and sulpher dioxide and R12. Ammonia was dropped due to its anti-social effect (it stinks) and explosive properties. R12 was used up until recently and still exists in older refrigeration systems. It use was stopped due its ozone depletion potential, in fact it's the benchmark against which all other substances are compared. Other substances that have been used as refrigerants include diluted anti-freeze, brine, some acids and carbon dioxide.
R12 and its colleague R22 where developed in the USA prior to World War II but did not reach the likes of the UK until after the war. R22 is still used today but R12 has been replaced by a slew of substance with similar properties which are still ozone depleters although to a lesser degree than R12.