A hand-held instrument for measuring the length of curved lines, especially on a map, usually having at its tip a small rotating wheel that is rolled along the line to be measured; a map-measurer.
Originally the wheel turned on a screw and had to be rolled backwards along a straight scale until it reached its former position on the screw. The reading is now shown directly on a dial or digital display.
Mid 19th century. From ancient Greek ὀπίσω backwards (from ὀπι- + a suffix forming adverbs indicating direction) + -meter.