An entangled state is a quantum state shared by two particles whose measurement outcomes are correlated.
A composite state is entangled when it cannot be written as a product of separate states for the individual particles. Measuring one member can reveal correlations with the other, even when the particles are far apart. The correlations are stronger than those allowed by classical local hidden-variable models, as demonstrated by Bell-test experiments.
Entanglement does not permit faster-than-light messaging. Each local measurement result is random, and the correlation becomes evident only when observers later compare records through ordinary communication. Entanglement is central to quantum teleportation, quantum cryptography and some quantum-computing protocols.