• a straight line with respect to a coplanar straight line when they have no common point or when they are merged;
• a straight line with respect to a plane when the plane contains a line parallel to the given line;
• a plane with respect to another plane when any line in one plane has a parallel line in the other plane

Note 1 to entry: Parallelism is an equivalence relation for two lines or for two planes.

Note 2 to entry: Usual wordings for lines are "a line A is parallel to line B", or "lines A and B are parallel", or " A and B are parallel lines". Similar wordings are used for the other cases.