 Solution

For a first-order theory $T$ in a first-order language $L$, a complete $n$-type is a maximal $T$-consistent set $p(x_1,\dots ,x_n)$ of formulas whose free variables lie among $x_1,\dots ,x_n$. Equivalently, it chooses exactly one of $\phi$ and $\neg \phi$ for every such formula while remaining consistent with $T$.

An isolated type $p$ is isolated by a formula $\theta (\bar{x})$ when $T\cup \{\exists \bar{x}\theta (\bar{x})\}$ is consistent andfor every $\phi \in p$. The type is an omitted type in an $L$-structure $M$ when no tuple $\bar{a}\in M^n$ satisfies every formula in $p$.

The omitting types theorem states that if $T$ is a consistent theory in a countable language and $(p_i{)}_{i\in \mathbb{N}}$ is a countable family of nonisolated finite-arity types, then $T$ has a countable model omitting every $p_i$.