Two vectors 
and 
are parallel if their cross product is zero, i.e.,
.
Parallel Vectors
See also
Cross Product, Parallel Lines, PerpendicularExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Parallel Vectors." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ParallelVectors.html