The hyperbola with equation \[\frac{x^2}{a^2}-\frac{y^2}{b^2}=1\] has foci at \((\pm ae, 0)\) and directrices \(x=\pm a/e\), where its eccentricity \(e\) is given by \(b^2 = a^2 (e^2 -1)\).
Glossary
Directrix of a hyperbola
The hyperbola with equation \[\frac{x^2}{a^2}-\frac{y^2}{b^2}=1\] has foci at \((\pm ae, 0)\) and directrices \(x=\pm a/e\), where its eccentricity \(e\) is given by \(b^2 = a^2 (e^2 -1)\).