In thermodynamics, ideal gas refers to a body of particles (atoms or molecules) in a state of gas particles, which are said to abide by the Boltzmann chaos assumption, i.e. have non-correlations of velocities, and to obey the ideal gas law:

$PV = nRT\,$

where P is the pressure, V the volume, n the number of particles , R the gas constant, and T the temperature of the body of gas. [1] Deviations in the behavior of gases, from the ideal gas law, occur at extreme pressures and temperatures, at which point the gas no longer is considered "ideal".

Perfect gas | Ideal perfect gas
The prefix term "ideal" is a modern etymological evolution stemming from the phrase "perfect gas" in the context of making a "perfect" vacuum via explosion in early gunpowder engine and other vacuum engine prototypes, such as is found in the works of Denis Papin and Christiaan Huygens.

In the early 20th century, terms such as “ideal perfect gas” (Preston, 1904) (Ѻ), were being employed.

References
1. Perrot, Pierre. (1988). A to Z of Thermodynamics (section: Ideal gas, pgs. 144-48). Oxford University Press.