What is the name for the distance between two foci within an ellipse?

The straight line between points F and F.
The straight line between points F' and F.

I can't find the answer to this question anywhere, and yes, this is important because it represents a gap in my knowledge, and that's just not acceptable for a being that likes to think of herself as being omniscient. ALL the other characteristics have designations (ex. minor axis, semi-major axis). Within the eccentricity equation it is designated by the letter c. e = c/2a, with a the semi-major axis, but of course that's not a name, just a letter (sorry Elon, but X is just a letter. You should have kept the name 'Twitter').

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27 d
Since it doesn't appear to have a name, I'm christening it now. I'm calling it the 'Bifocal Displacement'. Catchy name? Good. That's what it is. :)
What is the name for the distance between two foci within an ellipse?
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