Definition
Line segment
A line segment is a finite straight path joining two endpoints, with both endpoints included.
What it means
Mark two points, then draw the straight path joining them. That path stops at both points, so it is a line segment. A line continues forever in both directions, and a ray stops at only one end.
Remember it
Two endpoints, one straight piece.
Example
Points A and B are 7 cm apart. Segment AB is the straight piece joining them, and its length is 7 cm. Segment BA names the same segment because the endpoints are the same.
Watch for
A curved route from A to B is not a line segment, even if it starts and ends at the same two points.
Common mix-up
A line has no endpoints, a ray has one endpoint, and a line segment has two endpoints.
Exact meaning
A line segment is the set of all points on a straight line between two named endpoints, together with the endpoints. Unlike a line or ray, it has a finite, measurable length.