We found 7 dictionaries that define the word mass numbers:

General (4 matching dictionaries)

1. mass numbers: Merriam-Webster
2. mass numbers: Collins English Dictionary
3. mass numbers: Wiktionary
4. mass numbers: TheFreeDictionary.com

Computing (1 matching dictionary)

5. mass numbers: Encyclopedia

Medicine (2 matching dictionaries)

6. mass numbers: Merriam-Webster Medical Dictionary
7. mass numbers: Medical dictionary

(Note: See mass_number as well.)

Definitions from Wiktionary (mass number) ▸ noun:  (physics, physical chemistry) The total number of protons and neutrons in an atomic nucleus. Symbol: A ▸ Also see mass_number ▸ Words similar to mass numbers ▸ Usage examples for mass numbers ▸ Idioms related to mass numbers ▸ Wikipedia articles (New!) ▸ Words that often appear near mass numbers ▸ Rhymes of mass numbers ▸ Invented words related to mass numbers