Definition:Definable

Definition
Let $\mathcal{M}$ be an $\mathcal{L}$-structure with universe $M$.

Let $A$ be a subset of $M$.

and let $\bar b$ be a $n$-tuple of elements from $M$.

Let $\mathcal{L}_A$ be the language formed by adding constant symbols to $\mathcal{L}$ for each element of $A$.

$\bar b$ is definable over $A$ if there is an $\mathcal{L}_A$-formula $\phi(\bar x)$ with $n$ free variables such that the set $\{\bar m \in M^n : \mathcal{M}\models \phi(\bar m)\}$ contains $\bar b$ but nothing else.

Equivalent Definition
Using the same symbols as above:

$\bar b$ is definable over $A$ if every $A$-automorphism is an $A,b$-automorphism.

The proof of equivalence of these definitions can be found in Equivalence of Definitions of Definable.