ideal gas

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gas having particles that have perfectly elastic collisions and negligible volume and intermolecular forces, thus exactly obeying the ideal gas law. No known gas is an ideal gas, but actual gases approximate this behavior at relatively low pressure and high temperature.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics a hypothetical gas, whose molecules exhibit no interaction, and undergo elastic collision with each other and with the walls of the container

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a hypothetical gas with molecules of negligible size that exert no intermolecular forces