Definite

  1. No uncertainty
  2. No Spread(i.e., zero Variance)
  3. No probability distribution

An observable A^\hat{A} has a definite value aa on a state ∣ψ⟩\ket{\psi} when

A^∣ψ⟩=a∣ψ⟩\hat{A}\ket{\psi}=a\ket{\psi}

i.e., ∣ψ⟩\ket{\psi} is an eigenstate of A^\hat{A} with eigenvalue aa