A condition in which two or more particles are intrinsically parts of an inseparable quantum system. The individual constiuents or particles cannot be fully described or considered without taking into account the entire entangled quantum system. Measurement of one part of the entangled quantum system will "collapse" the system. Namely, locally interaction on one constituent will affect the other constituents, even if the pieces of the entangled system are separated by large distances. For instance, take the example of two entangled particles, A and B---that measuring a property of A (say, the particle’s polarization) is necessarily to know the corresponding property of B, even if you’re not there with a detector to observe B and even if the existence of that property had no prior fixed value until the moment particle A was detected.