first quantum numbern. The quantum number that specifies the size of the orbital of an electron in an atom and indicates in which shell the electron is located, beginning with the number 1 for the smallest shell, the orbital closest to the nucleus. Also called principal quantum number.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.