There was some experiment that I read about some time back in which two particles (or the same particle, but split into two) were sent in opposite directions, but when something happened to one, it happened to the other at the same moment, as if they were the same particle.
I thought the word for this was "entrainment", but looking that up does not lead back to this experiment.
What is the term for this, assuming that I am not remembering something that never happened.