Parallel Lines
two straight lines are parallel to each other when they lie in the same direction. Any number of straight lines are parallel to each other when they have the same direction, or when they are respectively parallel to a given straight line.
PARALLELOPIPED, PARAL- LELOPIPEDON, in geometry, a regular PARALLELOPIPED, or PARALsolid bounded by six plane surfaces, or parallelograms, the opposite pairs of which are similar, parallel, and equal to each other. If the parallelograms are squares, the solid is a cube.