planimeter
noun
pla·nim·e·ter plā-ˈni-mə-tər 
plə-
: an instrument for measuring the area of a plane figure by tracing its boundary line
Word History
Etymology
French planimètre, from Latin planum plane + French -mètre -meter
First Known Use
circa 1858, in the meaning defined above
Medical Definition
planimeter
noun
pla·nim·e·ter plā-ˈnim-ət-ər, plə-
: an instrument for measuring the area of a plane figure by tracing its boundary line