atomic number

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

• noun The number of protons in an atomic nucleus.

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

• noun physics, chemistry The number, equal to the number of protons in an atom that determines its chemical properties. Symbol: Z

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

• noun the order of an element in Mendeleyev's table of the elements; equal to the number of protons in the nucleus or electrons in the neutral state of an atom of an element