What Is Arc Length?
Arc length measures the distance along a curved section of a circle's circumference, rather than the straight-line distance between its endpoints. It depends on both the circle's radius and the size of the angle (central angle) the arc spans, since a bigger circle or a wider angle both produce a longer arc.
Example
A circle with a radius of 10 and a central angle of 90° (a quarter of the full circle) has an arc length of (90/360) × 2π(10) = (1/4) × 20π = 5π ≈ 15.7.