What is a line that intersects a circle or arc at only one point called?

A line that intersects a circle or arc at only one point is called a tangent. This definition stems from the geometric properties of circles. A tangent touches the circle precisely at one point, known as the point of tangency, and does not cross into the interior of the circle. This unique property distinguishes tangents from other lines related to circles.

In contrast, a diameter is a specific type of chord that passes through the center of the circle, dividing it into two equal halves. A chord is defined as any line segment whose endpoints lie on the circle, thus it intersects the circle at two points. A secant is a line that not only intersects the circle but does so at two distinct points, crossing completely through the circle.

Therefore, the characteristic of intersecting at only one point makes the tangent the correct choice in this context.