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A line segment is any line (straight or not) which terminates at two points.

Straight Line Segment

A straight line segment is a line segment which is straight.

In the words of Euclid:

A straight line segment can be drawn joining any two points.

(The Elements: Postulates: Euclid's Second Postulate)


Each of the points at either end of a line segment is called an endpoint of that line segment.

Similarly, the point at which an infinite half-line terminates is called the endpoint of that line.

In the words of Euclid:

The extremities of a line are points.

(The Elements: Book $\text{I}$: Definition $3$)


Let $L = AB$ be a line segment whose endpoints are $A$ and $B$.

Let $M$ be a point on $L$ such that the line segment $AM$ is equal to the line segment $MB$.

That is, let $M$ be the bisector of $L$.

Then $M$ is the midpoint of $L$.

Equality of Line Segments

Two line segments are equal if and only if they have the same length.