The Paris meridian is a meridian line running through the Paris Observatory in Paris, France – now longitude 2°20′14.02500″ East. It was a long-standing rival to the Greenwich meridian as the prime meridian of the world. The "Paris meridian arc" or "French meridian arc" (French: la Méridienne de France) is the name of the meridian arc measured along the Paris meridian.

Arc length is the distance between two points along a section of a curve.. Determining the length of an irregular arc segment by approximating the arc segment as connected (straight) line segments is also called rectification of a curve. If the rectification of a curve results in a finite number (so the curve has a finite length), then the curve is said to be rectifiable.