What term describes measuring the shortest distance from an inside object's perimeter to a containing outside object's perimeter?

Question

Given two closed curves where the first curve is completely inside the second curve, what term describes the minimal distance to travel from the inside curve to the outside curve? For example, in the following image, the first curve is an ellipse in green, the second curve is an ellipse in red, and the minimal distance is shown by the arrowed blue line.

I'm thinking the blue line could be called a minimal tangential distance since it it goes to the point perpendicular to the tangent at each end, however, a Google of that term gives things like tangential distance fields so that appears to be incorrect.

Is there a term for this shortest distance, and if so, what is it called?

You can consider the curves as subsets of the plane and in this respect, the distance is just the distance between the curves as sets. I don't think there is a specific term for it, but it becomes clear what you are talking about once you specify you are measuring the distance between subsets and not between its points. — Sak, Mar 23 '16 at 16:56
Related question: http://math.stackexchange.com/a/1511105/5886 — WilliamKF, Apr 06 '16 at 23:24

What term describes measuring the shortest distance from an inside object's perimeter to a containing outside object's perimeter?

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