Focus of a conic
= Focus of a conic
In the focus-directrix description of a noncircular <conic section>, the focus is the fixed point $F$ for which every point $X$ of the curve satisfies $|X-F|=e\,\operatorname{dist}(X,L)$, where $L$ is a <directrix of a conic> and $e$ is the <eccentricity of a conic>. For $y^2=4ax$ with $a>0$, the <focus of a conic> is $(a,0)$, the <directrix of a conic> is $x=-a$, and $e=1$.