The Physics of Quantum Information
Dirk Bouwmeester, Artur Ekert and Anton Zeilinger
Quantum Entanglement

Schrödinger coined the term ‘entanglement’ to describe this peculiar connection between quantum systems (Schrödinger, p. 555):

When two systems, of which we know the states by their respective representatives, enter into temporary physical interaction due to known forces between them, and when after a time of mutual influence the systems separate again, then they can no longer be described in the same way as before, viz. by endowing each of them with a representative of its own. I would not call that one but rather the characteristic trait of quantum mechanics, the one that enforces its entire departure from classical lines of thought. By the interaction the two representatives [the quantum states] have become entangled.
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