• a straight line with respect to another straight line when their associated vectors are orthogonal;
• a straight line with respect to a plane when the line is perpendicular to any line of the plane;
• a plane with respect to another plane when it contains a straight line which is perpendicular to any line of the other plane

NOTE Usual wordings for lines are "line A is perpendicular to line B", or "lines A and B are perpendicular", or " A and B are perpendicular lines". Similar wordings are used for the other cases.