Kinetic Molecular Theory defines ideal gas behavior. 

Depending on the textbook or website you look at, there may be a different number of postulates. Basically they are all the same, some sources combine different postulates into one or break some down further. Regardless of the number, here is what defines Kinetic Molecular Theory:

1. Gases are composed of particles that behave like hard, spherical objects in a state of constant, random motion. 

2. These particles are much smaller than the distance between particles. Most of the volume of a gas is therefore empty space.

3. These particles move in a straight line until they collide with another particle or the walls of the container. Collisions between gas particles or collisions with the walls of the container are perfectly elastic. None of the energy of a gas particle is lost when it collides with another particle or with the walls of the container. The pressure exerted by a gas in a container results from collisions between the particles and walls of the container. 

4. There is no force of attraction between gas particles or between the particles and the walls of the container.

5. The average kinetic energy of a collection of gas particles is directly proportional to the temperature of the gas and nothing else.