Since World War I, the U.S. and U.K. military have used the metric systems when performing combined operations with the French who used the metric system. The maps were made by the French and the term "kilometer" became part of the U.S. military lexicon after World War I.
The term "klick" is derived from the word "kilometer." So, one klick equals one kilometer.
Since World War II and the creation of NATO, all maps made and used by NATO members comply with the NATO Standardization Agreements. The Military Grid Reference System (MGRS) is the mapping system standard used by NATO military members for locating points on the earth and can pinpoint a place on Earth to the nearest meter.
But among members of the military, the term "klick" is a standard measure of walked distances. If a soldier radios "We're 10 klicks south of your position," that means they are 10 kilometers away, or 6.2 miles away.
Most foreign maps will have elevation contour lines measured in meters as well.
History of the Word "Klick"
Some military historians believe that the term originated in Vietnam with the Australian Infantry. As the story goes, infantry soldiers would navigate by bearing (compass direction) and would measure distance by pacing (this was, of course, prior to GPS devices).
In order to keep track of distance, one or two soldiers would be assigned to count their paces. About 110 paces on flat land, 100 paces down-hill, or 120 paces up-hill would equal 100 meters. The soldier would keep track of each 100-meter "lot" by moving the gas regulator on the Australian L1A1 rifle, one mark.
After moving it 10 marks (1000 meters), the soldier would signal the section commander using hand signals, then indicate movement of 1000 meters by lifting the rifle and rewinding the gas regulator with a movement of the thumb, resulting in an audible "click."
Non-Military Uses of "Click"
In military-speak, the term "click" (spelled with a "c" instead of a "k") is used when sighting-in a weapon, such as a rifle. On most weapons, one "click" equals one minute of arc, or — in other words, one inch of distance at one hundred yards. So, moving the site adjustments of the rifle "one-click" will change the point of impact one inch for a target 100 yards away, two inches for a target 200 yards away, and so forth.
For the detailed oriented, one Minute of Angle (MOA) at 100 yards is actually a tad over one inch (There are 360 degrees in a circle and each degree is divided into 60 minutes. If we round to the nearest 1/100 of an inch, at 100 yards one degree measures 62.83 inches. One MOA, 1/60 of that, measures 1.047 inches), but rounding it works for quick calculations. The term comes from the clicking sound made by the sight adjustment knobs as they are turned.
Latitude and Longitude vs. Grid Coordinates
Some American maps still also use the longitude and latitude system and continue to do so on the water.
The United States Military uses the MGRS which is measured in meters and the Latitude and Longitude are measured in statute miles.