Question

What is 2F called in simple words?

Answer 1

We have a relation that
radius of curvature (R)=2 times the focal length(F)

In geometry, the center of curvature of a curve is found at a point that is at a distance from the curve equal to the radius of curvature lying on the normal vector. It is the point at infinity if the curvature is zero. The osculating circle to the curve is centered at the centre of curvature.