pla·nim·e·ter (plə-nĭm'ĭ-tər, plā-) n. An instrument that measures the area of a plane figure as a mechanically coupled pointer traverses the perimeter of the figure. pla'ni·met'ric (plā'nə-mět'rĭk), pla'ni·met'ri·cal (-rĭ-kəl) adj., pla'ni·met'ri·cal·ly adv., pla·nim'e·try n.