Ch 2.5, Ch 6.2 A state $|\psi\rangle$ satisfying $\hat{A}|\psi\rangle = a|\psi\rangle$ for operator $\hat{A}$ with eigenvalue $a$. Measurement of $\hat{A}$ on an eigenstate always yields the result $a$ with certainty. *See also:* Eigenvalue, Spectral theorem, Observable.