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.