Which term describes two lines that run parallel but in opposite directions?

The term that describes two lines that run parallel but in opposite directions is "antiparallel." In geometry, two lines are considered parallel if they are always the same distance apart and never meet. When these lines also run in opposite directions, they are specifically referred to as antiparallel.

This concept is often employed in various fields, including physics and engineering, particularly when discussing vectors or the orientation of lines in space. For instance, in the context of vector analysis, antiparallel vectors have the same magnitude but point in diametrically opposite directions.

The other terms do not capture the specific nature of lines running in opposite directions while remaining parallel. "Perpendicular" describes lines that intersect at a right angle, "intersecting" refers to lines that cross at any angle, and "collinear" pertains to points or lines that lie on the same straight line without regard to their orientation.