A tangent. And it does not need to be a circle - it can be any curve.
The tangent.
It is a straight line that touches the circle.
A tangent of a circle is a straight line that touches the circle at only one point.
A straight line from the center of a circle to the outer edge is called a radius.
Tangent
The tangent.
Tangent.
A tangent is always perpendicular to the radius of a circle. A radius is a straight line going from the center of the circle to the circumference (edge) of the circle. A tangent is a straight line outside the circle that touched the circle at one (and only one) point. When a tangent touches the outside edge of the circle at the same point where a radius touches the edge of the circle, the angle between the radius and tangent line is 90 degrees meaning they are perpendicular.
It is a straight line that touches the circle.
It is the tangent of the circle
A tangent of a circle is a straight line that touches the circle at only one point.
A straight line from the center of a circle to the outer edge is called a radius.
A striaght line , outside the circle, that just touches the circle in one place.
Tangent
The curved line that traces the boundary or perimeter of a circle is called the circumference. A straight line that touches the circle at just one point is called a tangent. See the related link, 'The Parts of the Circle', below this answer.
It is the radius of a circle
A tangent of a circle is a straight line that touches the circle's circumference at just one point