the.com/mass number

means The total count of protons and neutrons in an atom's nucleus, written as the whole number attached to an element's name — like carbon-14's 14.

from A 20th-century coinage from physics, built simply from 'mass' (Latin massa, a lump or dough-like substance, itself from Greek maza) plus 'number.' It arrived alongside the early discoveries of isotopes and the nucleus, when scientists needed a plain term for the near-whole-number mass they kept measuring relative to hydrogen — no poetry intended, just protons and neutrons tallied up.