Originally a term for X-Rays scattered by atomic nuclei. Rutherford conducted an experiment to infer the size of the atomic nucleus from BackScatter, and various machines use X-Ray BackScatter to examine an object or person.
Similarly, in computer lingo it is a term for the packets scattered back during a spoofed DenialOfService attack. A host trying to reply to packets sent to it with a spoofed source address will (attempt to) send them to random hosts on the InterNet (the recipients may not even exist). By "listening" to these packets you can determine who is being DDoSed.