Definition 17.5. Let $M\models T$, $A\subseteq M$. Then $b\in \mathrm{acl}(A)$ if there is an $L_A$-formula $\varphi (x)$ such that

$$M\models {\exists }_x^{=n}\varphi (x)$$

and $M\models \varphi (b)$.