The Einstein-Podolsky-Rosen (EPR) paradox is a thought experiment proposed in 1935 by Albert Einstein, Boris Podolsky, and Nathan Rosen to challenge the completeness of quantum mechanics. They argued that if two particles are entangled, measuring one particle instantly affects the other, regardless of the distance between them. This phenomenon, known as quantum entanglement, seemed to imply "spooky action at a distance," which contradicted the principles of local realism. The EPR paradox raises questions about the nature of reality and the limits of what can be known. It suggests that if quantum mechanics is complete, then particles do not have definite properties until measured, leading to debates about the interpretation of quantum theory. This paradox has inspired further research and discussions in the field of quantum physics.