Cyclometer: an instrument used to measure circular arcs.
Like you, I have no idea what that actually means.
However, I do understand this. A cyclometer is an instrument you can attach to a bicycle to measure the distance you travel.
Since biking is my favorite pastime, I know what a cyclometer is. Never have used one, though. I always wait until the next morning. When I can barely stand up, I am able to measure the distance traveled.
Cyclometer comes from the Latin words κύκλος (kuklos), a circle, a wheel, and μέτρον (metron), measure.