Click or Klick
This term became popular during the Vietnam War and is an indication of distance, a kilometer to be exact. The range of the big artillery guns was managed by turning one of two dials (for lack of a better term). One of which was for large adjustments, one click of the dial equaled one kilometer. The other was for smaller adjustments, less than a kilometer.
Some have indicated that the term predates Vietnam and could be from the Korean War or in the interwar period. It may, but it definitely came into popular use during the 60’s.