Entanglement is a phenomenon in quantum mechanics where two or more particles become correlated in such a way that their quantum states become interdependent, regardless of the distance between them. In other words, the properties of entangled particles are linked in a way that measuring the state of one particle can instantaneously affect the state of the other, regardless of the spatial separation between them. This instantaneous correlation is what Einstein famously referred to as "spooky action at a distance."
When particles are entangled, their quantum states become entangled as well. The quantum state of a particle is described by a mathematical construct called a wavefunction, which contains information about various properties of the particle. When particles are entangled, their wavefunctions become entangled, resulting in a joint or composite wavefunction that describes the entangled system as a whole.
The phenomenon of entanglement has been experimentally confirmed through numerous experiments, such as the famous Bell's theorem experiments. It has profound implications for our understanding of quantum mechanics and has applications in various fields, including quantum computing, quantum communication, and quantum cryptography.
It is important to note that entanglement is a purely quantum mechanical phenomenon and does not have an analog in classical physics. In classical physics, particles are considered to have well-defined properties that are independent of each other. However, in the quantum realm, particles can exhibit this intriguing entanglement property, which is a fundamental aspect of quantum theory.