|me]zō, |mē], ]sō+\ noun
Etymology: New Latin, from mes- + thorium
: 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 MsTh 1 or Ra 228
b. : an isotope of actinium — called also mesothorium 2 ; symbol MsTh 2 or Ac 228