An altimeter was an instrument used to measure the altitude to a fixed level. A RADAR altimeter made this determination by measuring the distance between the craft in flight and ground directly below it.
In 2254, the Talosians scanned the library computer banks of the USS Enterprise, including a diagram of the US Ranger 5 space probe. Among the equipment noted on the schematic was the probe's RADAR altimeter. (TOS: "The Cage")