The Bell's states or EPR pairs: 25  are specific quantum states of two qubits that represent the simplest examples of quantum entanglement; conceptually, they fall under the study of quantum information science. Wikipedia

quantum state of two qubits that exhibits maximal quantum entanglement Wikidata

One of a set of four entangled states, namely | Φ + ⟩ = 1 2 ( | 00 ⟩ + | 11 ⟩ ) , | Φ − ⟩ = 1 2 ( | 00 ⟩ − | 11 ⟩ ) , | Ψ + ⟩ = 1 2 ( | 01 ⟩ + | 10 ⟩ ) , and | Ψ − ⟩ = 1 2 ( | 01 ⟩ − | 10 ⟩ ) , where | x y ⟩ := | x ⟩ ⊗ | y ⟩ ; these are the simplest examples of entangled states. Wiktionary