Find the odd number / letters / word from the given alternative.

The odd one out is diagonal.

An arc is a part of the circumference of a circle. A diameter is a straight line passing through the center of a circle and two points on the circumference. A radius is a line segment from the center of a circle to a point on the circumference. A tangent is a line that touches a circle at exactly one point.

A diagonal is a line segment that joins two non-consecutive vertices of a polygon. In a square, all the diagonals are equal in length. In a rectangle, the diagonals are not equal in length.

In the given options, all the other options are related to a circle, while diagonal is not related to a circle.