In thermodynamics, the air-standard assumptions are a series of assumptions used to analyze cycles which may otherwise be too complicated. They are:

The working fluid is air which behaves ideally.

All processes in the cycle are internally reversible.

All combustion processes may be modeled by heat addition processes from an external source.

All exhaust processes may be modeled by heat rejection processes to an external source.

The air-standard assumptions are often abbreviated to ASA. There is also a fifth assumption which states that specific heats may be considered to be constant and evaluated at room temperature for the cycle. When this fifth assumption is used, the air-standard assumptions are called the cold air-standard assumptions. In general this fifth assumption is only valid for cycles at lower temperatures since specific heats are a weak function of temperature.

Source: Cengel and Boles, Thermodynamics: An Engineering Approach. Chapter 9, Gas Power Cycles