Theorem 17.3 (Fundamental Theorem of Stability (light)). The following are equivalent:

• (i)
  $T$ is stable.
• (ii)
  No $L$-formula has the order property with respect to $T$.
• (iii)
  For any $M\models T$, every $p\in S_n(M)$ is definable.
• (iv)
  Non-forking is an independence relation.