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

Intriguing point!

?

Is there a term for the centre to focus distance... , perhaps?

You'd have thought mathematicians would have invented a name for it by now!

I've never heard a word for it. "Focal distance" means something entirely different.

It's interesting that there is no simple formula to calculate the arc length L of an ellipse.
When I was still a girl I thought L should be equal to
L = π * (a+b),
where a and b are the half-axes; because I thought that, for a circle,
L = 2πr = π * (r+r)
so for an ellipse it should be π * (a+b).
I was hugely disappointed when I learned that my naïve approach was totally wrong... 😕

I nominate 'transverse differential' as the term.

Does this answer the question at hand?

The distance between the foci is equal to the length of the major axis. The distance between the center and either focus is called the eccentricity, denoted by e.

Update: Nope, just QA'd this, right answer, wrong question.

It is called a "directrix"

According to the Wikipedia article on an ellipse, it is twice c, where c is the "focal distance", or else the "linear eccentricity".

The distance between the two foci of an ellipse is called the focal length or linear eccentricity.

I am sure it doesnt have a name. We can just call it 2ae.

"Focius" also may work, it's short and easy to remember, sounding as "radius"

Could you go slower please?