A line segment is precisely defined using the undefined terms point and plane. These three terms are undefined terms in geometry. A point has no dimension. A plane has no thickness but extends at all directions. A line has no thickness but extends in one dimension.
In Geometry, we define a point as a location and no size. A line is defined as something that extends infinitely in either direction but has no width and is one dimensional while a plane extends infinitely in two dimensions. There are three undefined terms in geometry. And the third undefined term is the line.