Definition of mesothorium
: either of two radioactive products intermediate between thorium and radiothorium in the thorium series or a mixture of the two products obtained usually from thorium minerals (as monazite sand) and used as a substitute for radium especially in luminous paints:
a
: an isotope of radium — called also
mesothorium 1
—symbol MsTh1 or Ra228
b
: an isotope of actinium — called also
mesothorium 2
—symbol MsTh2 or Ac228
