Quantum entanglement is a phenomenon in quantum mechanics where two or more particles become intertwined in such a way that their states cannot be described independently of each other, even when they are separated by large distances. This means that measuring the state of one particle instantaneously determines the state of the other particle, regardless of the distance between them. This phenomenon challenges classical notions of causality and has been studied extensively in the field of quantum physics for its potential applications in areas such as quantum computing, quantum cryptography, and quantum teleportation.