The Einstein-Podolsky-Rosen (EPR) paradox is a thought experiment proposed in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen. It challenges the completeness of quantum mechanics by illustrating a scenario where two particles become entangled, meaning the state of one particle instantaneously affects the state of the other, regardless of the distance separating them. This phenomenon, known as quantum entanglement, raises questions about the nature of reality and the concept of locality. The EPR paper argued that if quantum mechanics were complete, it would imply "spooky action at a distance," which contradicted classical intuitions about how information should travel.