A click, or "klick" to use the spelling generally utilized in the U.S. military, is equal to a distance of 1 kilometer, or .062 miles. According to About.com U.S. Military, this term came into widespread use during the Vietnam War.
When sighting in a weapon, the term "click" is used to indicate one minute of arc. In this sense, a click is equal to 1 inch of distance at a range of 100 yards. In this usage, the term "click" has a clearly onomatopoetic derivation, as the sight adjustment knobs of a weapon make a clicking noise as they are turned.