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The Fundamental theorem of stability says that, for a complete theory with infinite models, the following are equivalent:

Choose $b\in N^m$ and an $L$-formula $\phi (\bar{x};y)$ such thatLet $p(y)=\mathrm{tp}(b/M)$. Stability and the Fundamental theorem of stability make this a definable type. Apply its definition to the formula $\psi (y;\bar{x})=\phi (\bar{x};y)$. There is an $M$-formula $d_p\psi (\bar{x})$ such that, for every $\bar{a}\in M^n$,Thus $d_p\psi$ defines $X\cap M^n$ in $M$, which is the stable trace of a definable set property.