Question

The part of a line between two points on the line

Answer 1

A 'line' is infinite in both directions.

If you mark one point on it and say that's an end, then the rest of it
is still infinite in the other direction, and it's called a 'ray'.

If you mark two points on it and say that those are both ends,
then it has a definite length, and it's called a 'line segment'.