Mass number (Chemistry Dictionary)

Mass number (A) : Also atomic mass number or nucleon number.

The total number of protons and neutrons (together known as nucleons) within the nucleus of an atom. It determines the atomic mass of the atom. Mass number varies between different isotopes of the same chemical element, and is often included either after the element’s name (as in carbon-12) or as a superscript to the left of the element’s symbol (as in ¹²C) to identify a specific isotope.