A rangefinder is, in general, a device that allows one to measure/estimate the distance between the point A where he is and another point B that is visible from A. There are various sorts of rangefinders, the most common ones are:

  1. optical rangefinders, used from WWI to range guns on the battlefield, at sea and even for AA fire; another widespread application is cameras, famously the rangefinder Leicas.
  2. ultrasound rangefinders, with applications ranging from substituting tape measures to autofocus systems on Polaroid cameras
  3. laser rangefinders, used for training golf players.
There are also more exotic rangefinding systems based on measuring the time of flight of radar pulses, but they are not into wide use yet. There is a web page about them at http://www.drykiln2000.com/WAP/mir/mir.htm.

Additionally, more than you ever wanted to know about time of flight measurement techniques at: