What do you call a line, segment, or ray that intersects a circle exactly once?

A line, segment, or ray that intersects a circle exactly once is known as a tangent. This is because a tangent touches the circle at one specific point without crossing it. The point of contact is where the tangent and the circle meet. Tangents are significant in geometry because they reveal properties related to circles and their intersections with other geometric figures.

In distinguishing this from other terms: a chord spans across a circle, connecting two points on the circumference; a diameter is a specific type of chord that passes through the center of the circle; and an arc refers to a portion of the circumference of the circle itself. Thus, the defining characteristic of a tangent — its unique ability to contact the circle at only one point — clearly establishes its role as the correct answer.