Backflow - A reverse flow of water or liquids, caused by negative pressure
Backflow preventer - is a device that prevents the black flow of potable (drinking) water. There are many type, sizes and used for diffrent things. A air gap is the best. http://www.plumbingsupply.com/backflow.html .
Back water valve - is a device that prevents the back flow of waste water. There are many types and sizes, make of material. Over head sewer beats all back water valves.
Back Pitch - is a pipe that is angled the wrong way.
Belly - A sagging pipe
Branch - a horzintal section of pipe comming from the main. Could be waste or supply
