mass number mass noun • mass observation · · · Info · Definitions · Synonyms · Characters · · mass number · · Text Common · · Latin · · Scripts 11 · · Length 3 · · Syllables Definitions · · · (noun) the sum of the number of neutrons and protons in an atomic nucleus. Synonyms: mass number, nucleon number. · · Noun Definition 1 Synonyms · · · nucleon number · · Synonym 1 Characters · · space U+0020 a latin small letter a U+0061 b latin small letter b U+0062 e latin small letter e U+0065 m latin small letter m U+006D n latin small letter n U+006E r latin small letter r U+0072 s latin small letter s U+0073 u latin small letter u U+0075