Ch 11.4 A state of a composite system that can be written as a product: $|\psi_{AB}\rangle = |\phi_A\rangle \otimes |\chi_B\rangle$ (pure), or $\hat{\rho}_{AB} = \sum_i p_i\hat{\rho}_A^{(i)}\otimes\hat{\rho}_B^{(i)}$ (mixed). A state that is not separable is entangled. *See also:* Entanglement, Tens