Omnidirectional Vs. Unidirectional
http://www.lockergnome.com /media/2011/08/30/omnidirectional-vs-unidirectional/
Omnidirectional devices broadcast or receive in all directions. Unidirectional devices only work in one direction, or work best in a limited portion of the directions. This lets them direct the signal more efficiently, but only in one direction that has to be pointed.