Quantum entanglement is a physical phenomenon where a pair or group of particles interact in ways such that the quantum state of each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance. Measuring one particle immediately affects the state of the other, faster than the speed of light, though no information is transmitted.
Described by Einstein, Podolsky, and Rosen (EPR paradox) in 1935 as 'spooky action at a distance', intended to show the incompleteness of quantum mechanics. It was later experimentally verified by John Bell (Bell's theorem) and Aspect, Clauser, and Zeilinger (who won the 2022 Nobel Prize).
Entanglement is the basis for emerging technologies like quantum computing (entangled qubits), quantum cryptography (QKD for secure communication), and quantum teleportation. It challenges our intuitive understanding of locality and reality.