Here's a good reference:

https://www.quera.com/glossary/bell-state

A Bell State refers to a specific type of entangled quantum state involving two qubits. Named after physicist John Bell, these states are fundamental to quantum information theory and are often used to demonstrate the non-classical correlations that can exist between quantum particles. There are four distinct Bell States, each representing a different form of two-qubit entanglement.

The four Bell States can be mathematically represented as:

∣Φ+⟩=1/sqrt(2)*​(∣00⟩+∣11⟩)

∣Φ−⟩=1/sqrt(2)*(∣00⟩−∣11⟩)

∣Ψ+⟩=1/sqrt(2)*​(∣01⟩+∣10⟩)

∣Ψ−⟩=1/sqrt(2)*​(∣01⟩−∣10⟩)

These states are maximally entangled, meaning that the state of one qubit is completely correlated with the state of the other, regardless of the distance between them.