Parallel lines
Two or more coplanar lines that never cross or intersect.

Parallel planes
Planes that have no points in common.

Perpendicular lines
Two segments, rays, or lines that form a 90 degree angle.

Perpendicular planes
Planes in which any two intersecting lines, one in each plane, form a right angle.

Skew lines
Non-coplanar lines that don't intersect.

Slope
The measure of the tilt of a line; rise over run.