In mathematics, a cardinal number κ > א0 is called strongly inaccessible iff the following conditions hold:
- κ is regular; that is, cf(κ) = κ.
- κ is a strong limit cardinal, that is, 2λ < κ for all λ < κ.
Under the Generalized Continuum Hypothesis, a cardinal is strongly inaccessible iff it is weakly inaccessible.