atomic number

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. The number of protons in an atomic nucleus.

Wiktionary

1. n. The number, equal to the number of protons in an atom that determines its chemical properties. Symbol: Z

WordNet 3.0

1. n. 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