Explain why line segment is considered a defined term in geometry.

The defined terms in geometry are terms that can be combined either with other defined or undefined terms. When they are combined, another term is generated.

A line segment is a line joining between two points along with all the points between them.

Combining two segments can generate other terms, for example,
combining line segments perpendicularly together will generate a right angle.

That's why line segments are considered defined terms in geometry.