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Two model-theoretic characterisations of inaccessibility
Firstly, a cardinal κ is inaccessible if and only if κ has the following reflection property: for all subsets U ⊂ Vκ, there exists α < κ such that (V_α,∈ ,U∪ V_α) is an elementary substructure of (V_{κ },∈ ,U). (In fact, the set of such α is closed unbounded in κ.) Equivalently, κ is Π _{n}^{0}-indescribable for all n ≧ 0.
It is provable in ZF that ∞ satisfies a somewhat weaker reflection property, where the substructure (Vα, ∈, U ∩ Vα) is only required to be 'elementary' with respect to a finite set of formulas. Ultimately, the reason for this weakening is that whereas the model-theoretic satisfaction relation ｜＝ can be defined, truth itself cannot, due to Tarski's theorem.
Secondly, under ZFC it can be shown that κ is inaccessible if and only if (Vκ, ∈) is a model of second order ZFC.
In this case, by the reflection property above, there exists α < κ such that (Vα, ∈) is a standard model of (first order) ZFC. Hence, the existence of an inaccessible cardinal is a stronger hypothesis than the existence of a standard model of ZFC.

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