In black hole physics and GR, what does the term back reaction means?
A binary system, especially one of high gravitational field such as the binary pulsar PSR 1913+16, radiates gravitational waves that carry energy and momentum away from the system. This radiation by Newton's Third Law produces a back reaction that has to be taken into account when evaluating the detection of any gravitational waves coming from that system. See this article from "The Confrontation between General Relativity and Experiment" by Clifford M. Will
Gravitational radiation back-reaction.
There is also a back reaction to Hawking radiation produced by the zero point energy field at the event horizon. Influence of the back reaction of the Hawking radiation upon black hole quasinormal modes
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