The gas or mixture of gasses in a furnace surrounding components during heat treatment.

The nature of the heat treatment atmosphere varies according to the process being carried out and can be inert (completely unreactive, e.g. argon); neutral (does not change the composition of the component but may protect it from oxidation or other unwanted reactions, e.g. hydrogen) or reactive (plays an important part in the heat treatment by controlling or altering the composition of the component surface, e.g. endothermic atmosphere).