valence electron

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

• noun An electron in an outer shell of an atom that can participate in forming chemical bonds with other atoms.

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

• noun chemistry any of the electrons in the outermost shell of an atom; capable of forming bonds with other atoms

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

• noun an electron in the outer shell of an atom which can combine with other atoms to form molecules