Consider a cycloid traced by a point $P$ on the circumference of a circle.
An arc of the cycloid is defined as being a part of the cycloid traced out by $P$ between the points where it meets the straight line.
Some sources give this as an arch, while others call it a hump.
Weisstein, Eric W. "Cycloid." From MathWorld--A Wolfram Web Resource. http://mathworld.wolfram.com/Cycloid.html