The mathematical terms that cannot be defined for all cases in a system of geometry are the parallel lines and midpoint of a line segment. A line is an infinite series of points in a row and it does not occupy any space. They are also called one dimensional since the direction is in one dimension. A point has a location but no specific size. It can describe a location but you can never determine its actual size. Imagine the point at the end of the previous sentence. And then imagine it getting smaller and smaller until it disappears. We can only use a dot to represent the point.