(1) The basic unit of length of the metric system (39.37 in.). Originally defined as one ten-millionth of the distance from the equator to the North Pole. Now defined as 1,650,763.73 wavelengths of the orange-red line of the isotope krypton 86. (2) A device for measuring the flow rate of liquids, gases, or particulate solids, e.g., flowmeters, rotameters, proportioning equipment.See Gauge.