Quantum entanglement is a physical phenomenon that occurs when two or more particles interact in such a way that the quantum state of each particle cannot be described independently of the others, even when separated by a large distance. This means that the state of one particle can be affected by the state of another, even if they are separated by a large distance. Entanglement has been used in quantum computing and quantum cryptography, and has been proposed as a resource for other applications. It is also an important concept in quantum mechanics, and has been used to explain phenomena such as superconductivity and superfluidity.