A part of a circle is called an arc.

Let us take two points on the circle - A & B.

If we join A & B along the circle (the blue part), we get a part of the circle.

This part of the circle is called arc. We call it arc AB

Note: We can also join it from the other side, then it will be a major arc.Major arc is greater than 1/2 of the circle, minor arc is less than 1/2 of the circle