All right, so let's make sure that we're on the same page as to what we're talking about.
I assume we're referring to two-dimensional Euclidean space, yes? A plane that can be mapped out in Cartesian coordinates?
If so, then a point is defined as a specific location on the plane, which can be uniquely determine by its coordinate, such as (3,5).
A line is defined as the set of all of the points whose coordinates obey a specific algebraic relationship, namely y = a*x + b for some set values of a and b.
A line segment is defined as the set of all of the points within a given line that fall between two established points -- all (x,y) where y = a*x + b, c<=x and x<=d for some set values of a, b, c, and d.
Are we in agreement on the above definitions?