The phenomenon where two particles become correlated so that measuring one affects the other is called quantum entanglement.
In an entangled state, the particles must be described as one combined quantum system, even when they are separated by a large distance. Measuring a property such as spin or polarization on one particle can establish a corresponding result for the other. The individual result is still fundamentally random, so entanglement cannot be used to send messages faster than light.
Albert Einstein, Boris Podolsky, and Nathan Rosen highlighted the puzzle in their 1935 EPR paper. Erwin Schrödinger soon described the effect as entanglement and recognized it as a central feature of quantum theory. Later experiments testing Bell inequalities showed that these correlations cannot generally be explained by ordinary classical hidden variables.
Entanglement is often confused with quantum teleportation. Teleportation uses entanglement as a resource to transfer an unknown quantum state, but it does not transport matter instantly. Decoherence, by contrast, is the loss of quantum coherence through interaction with the environment.