 \(\{\phi_i \approx \psi_i: i \in I\} \vDash_{\bK } \phi \approx \psi \txtiff\)for every \(\bA \in \bK\) and every valuation \(v\) on \(\bA\), if \(\bv(\phi_i) = \bv(\psi_i)\), for all \(i \in I\), then \(\bv(\phi) = \bv(\psi)\).