[mez-oh-thawr-ee-uh m, -thohr-, mes-, mee-zoh-, -soh-]
- one of two radioactive decay products of thorium, an isotope of radium or actinium.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
- physics obsolete either of the two radioactive elements which are decay products of thorium. Mesothorium I is now called radium-228. Mesothorium II is now called actinium-228