The story
From meso- + thorium. Probably so-called because these two isotopes (²²⁸Ra and ²²⁸Ac) are, in the decay chain, intermediate between normal thorium (²³²Th) and radiothorium (²²⁸Th).
Etymology from Wiktionary, CC BY-SA 4.0.
mesothorium
Today it means: Certain members of the decay chain of thorium, mainly isotopes of radium and actinium Full definition →