“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 (or -functions) have become entangled.”

-E. Schrödinger

Learn more about Quantum Entanglement in this article.

Ref:

E. Schrödinger, “Discussion of Probability Relations between Separated Systems”, Mathematical Proceedings of the Cambridge Philosophical Society, Vol. 31, Issue 04, (1935).