Klick is used as both, unit of length and unit of velocity. It is a military slang adopted by military organizations across The Sol System. When used as length, 1 Klick equals 1 kilometer, when used as velocity, 1 Klick equals 1 kilometer per hour.
First uses of a word Klick were observed during the Vietnam War, where Australian troops used the click their rifles made while reloading to signal that they have walked roughly 1000 meters in a straight line, in order to measure distances. 
The term was soon adopted by United States military forces.
Usage in the Expanse seriesEdit
Klick is used aboard most military and some non-military spaceships to indicate the distance to the observed object. It is often used with distances exceeding one million kilometers. When talking about even greater distances, Astronomical unit (AU) is usually used instead.
|“||Before the radio cut out, I got authorization to fire if they come within half a klick of our position.||„|
| — Lieutenant Givens|
in Caliban's War