What does it mean when we say an iso curve has no interior? From the paper: "Standard Auctions with Financially Constrained Bidders" - by Che and Gale. The authors describe an isobid curve as the curve that represents payments of the same value in a 2-dimensional plane (wealth and valuation). They state that:

"The isobid curves are negatively sloped and have no interior"

What does it mean that an iso curve has no interior?
From Wikipedia:


  In mathematics, specifically in topology, the interior of a subset S of points of a topological space X consists of all points of S that do not belong to the boundary of S. A point that is in the interior of S is an interior point of S. [...] If S is a subset of a Euclidean space, then x is an interior point of S if there exists an open ball centered at x which is completely contained in S. (This is illustrated in the introductory section to this article.)