The distance between the two foci of an ellipse is called... what?

I know all about the axes, both minor and major, about eccentricity, the semi-latus rectum (no, that's got nothing to do with Uranus), and all else there is to know; except this. What's it actually called? Not how to calculate it, nor the symbol for it, but the name (if it even has one).


If it doesn't have a name, I propose that we Christen it the 'Bi-Focal Displacement'. :)