A hyperbola is a curve where the distances of any point from a fixed straight line (the

directrix) is always in the same ratio. That straight line is directrix of the hyperbola.

The equation of hyperbola is x² /a² - y² /b² = 1

Now, the equation of directrix is:

x = ±a/e   

where e = √{(a² + b² )/a² } is called eccentricity of hyperbola.