energy level

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

• noun The energy characteristic of a stationary state of a physical system, especially a quantum mechanical system.
• noun The stationary state of a quantum mechanical system.

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

• noun physics any of the discrete stable energies that a quantum mechanical system (such as the electrons of an atom) can have

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

• noun a definite stable energy that a physical system can have; used especially of the state of electrons in atoms or molecules