Interior
The interior of a set is the union of all its open subsets.
More informally, the interior of geometric structure is that portion of a region lying "inside" a specified boundary. For example, the interior of the sphere is an (open) ball and the interior of a circle is an (open) disk.
See also
ExteriorExplore with Wolfram|Alpha
WolframAlpha
More things to try:
Cite this as:
Weisstein, Eric W. "Interior." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Interior.html