What characterizes a line segment?

A line segment is characterized as a part of a line that has two distinct endpoints. This definition indicates that it has a definite length, as it stretches from one endpoint to the other. Unlike a full line, which extends infinitely in both directions and does not have any endpoints, a line segment is confined between its two endpoints.

The concept of length in a line segment is crucial for many geometric applications, making it a fundamental element in understanding shapes and figures in mathematics. Each endpoint represents a specific position on the line, allowing calculations like distance to be made easily, which is a key aspect of geometry.

The other provided options describe characteristics that do not apply to a line segment: lacking endpoints would classify it as a line, being infinite in length refers to a different type of geometric line, and being perfectly vertical pertains to a line's orientation rather than defining what a line segment is. Thus, the characteristic of having two endpoints is essential to the definition of a line segment.