What is Quantum Entanglement?

Introduction to Quantum Computing

"The phenomenon whereby a pair of particles are generated in such a way that the individual quantum states of each are indefinite until measured, and the act of measuring one determines the result of measuring the other, even when at a distance from each other."

"The quantum states of two objects have to be described with reference to each other, even though the individual objects may be spatially separated." Said differently, each particle of the group cannot be described independently of the state of the others, including when the particles are separated by a large distance.