An arc is the part of the circle between two points. Arcs are generally associated with the central angles they make and their measure is expressed as an angular measure using the measure of the central angle.
Arc length is a linear measure of an arc measured along the circumference of the circle.
Arc Length
As arc is a part of the circle, arc measure is a fraction of the circumference of the circle. Arc length is generally indicated by the letter 'l'.

Arc length is a linear measure of the arc measured along the circle.
It can be understood, that the arc length is a fraction of the circumference of the circle.
To measure the arc length physically:
  1. Place a thin wire or thread carefully along the circle.
  2. Mark the end points of the arc on the wire or thread.
  3. Stretch the wire/thread and measure the distance between the points marked using a Straight edge.