(related to Definition: 1.02: Line, Curve)
A line is a space of one dimension. If it had any breadth, no matter how small, it would be space of two dimensions, i.e. a surface; and if in addition, it had any thickness it would be space of three dimensions, i.e. a solid; hence a line has neither breadth nor thickness.
The modern definition of a line is a combined set-theoretic and topologic one. A line has been defined as a subset of another set fulfilling the topological property that if we take away a point from it, it will be disconnected. Connectivity is a topological property.