One "Klick" equals one Kilometer. In military terms, a "klick" means a distance of 1000 meters (one kilometer, or .62 miles) "People tend to agree the common use of "klick" as a distance dates back to the early 1960s. Some people say it became common among American troops who were serving in Germany at that time while others attribute it to troops serving in Vietnam alongside troops from Australia. Anyway, the bottom line is that "klick" is our military's slang for kilometer. What is interesting is that "click" is a shorthand term among military marksmen for setting the sights on their rifles. The use of "click" comes from the sound made as one adjusts the rifle's sights for range (that's distance for those who don't speak military " http://www.news-journal.com/featr/content/features/stories/2008/08/26/08262008_answerline.html
"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 second 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 1,000 yards away, two inches for a target 2,000 yards away, and so forth. The term comes from the clicking-sound made by the sight adjustment knobs as they are turned." U.S. Military FAQ -- How Far is a "Klick" in the Military?
One 'mil' is 1/1000 (millesime) of a radian. That is, one mil = an arc of 1meter over 1,000m distance. Really a very convenient way to define arcs. A deflection of 200m at 5000m distance would be 40mil. (also works for inches, yards, leagues, parsecs, you name it )
