Parallel lines requires the terms line and plane because line and plane that are parallel cannot cross each other therefore they will never make a point of intersection. The characteristics of that parallel lines consists of lines and planes. Perpendicular lines requires the terms line and point because whenever two lines intersect, it creates a point and whenever two planes intersect, it creates a line. The characteristics of that perpendicular lines are points and lines.
Parallel lines never intersect, but they must be in the same plane. The definition does not require the undefined term point, but it does require plane. Because they intersect, perpendicular lines must be coplanar; consequently, plane is not required in the definition.
