mesothorium

[ mez-oh-thawr-ee-uh m, -thohr-, mes-, mee-zoh-, -soh- ]
/ ˌmɛz oʊˈθɔr i əm, -ˈθoʊr-, ˌmɛs-, ˌmi zoʊ-, -soʊ- /

noun Chemistry.

one of two radioactive decay products of thorium, an isotope of radium or actinium.

British Dictionary definitions for mesothorium

mesothorium
/ (ˌmɛsəʊˈθɔːrɪəm) /

noun

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