The inner or enclosed surface of something WordNet 3.0 & Open English WordNet

In mathematics, specifically in topology, the interior of a subset S of a topological space X is the union of all subsets of S that are open in X. A point that is in the interior of S is an interior point of S. The interior of S is the complement of the closure of the complement of S. In this sense interior and closure are dual notions. Wikipedia

Mathematical concept that includes, for example, the inside of a shape Wikipedia Disambiguation

Given a subset S of a topological space X, the biggest set of points in S not part of the boundary of S Wikidata

Within the interior of something, closest to the center or to a specific point of reference. OmegaWiki

The set of all interior points of a set. Wiktionary

The set of all interior points. Wiktionary (translation)