planchet
noun
plan·chet ˈplan-chət 
1
: a metal disk to be stamped as a coin
2
: a small metal or plastic disk
Word History
Etymology
diminutive of planch flat plate, from Middle French planche
First Known Use
1611, in the meaning defined at sense 1
Medical Definition
planchet
noun
plan·chet ˈplan-chət
: a small metal disk or plate on which a sample of radioactive material is placed (as for quantitative determination of its activity)