Klick is military slang for kilometer.[1] The term was commonly used by United States soldiers during the Vietnam War in the 1960s and 1970s, although veterans recall its usage as early as the 1950s; this is attested by Robert Heinlein's use of the term in his 1959 novel Starship Troopers. Some claim that the term originated from the sound military vehicle odometers made as they turned over each kilometer. Another theory is that it is an example of condensed pronunciation or contraction of the term kilometer.