Let $F_1$ and $F_2$ be two fixed points (the foci).
All the points $P$ such that $\overline{PF_1}+\overline{PF_2}$ remains constant will form an ellipse.
All the points $P$ such that $\overline{PF_1}-\overline{PF_2}$ remains constant will form an hyperbola.
Is there a name for the constants $\overline{PF_1}\pm\overline{PF_2}$?
I should perhaps add that for horizontal ellipses, that constant's value is $2a$ and for vertical ellipses, it is $2b$ if the ellipse's equation is:
$\dfrac{(x-h)^2}{a^2}+\dfrac{(y-k)^2}{b^2}=1$
I would like to refer to that constant without having to explain everytime where it comes from.