In geometry, a flag is a sequence of faces of a polytope, each contained in the next, with exactly one face from each dimension. Wikipedia

Part of a polygon, polyhedron or higher polytope Wikipedia Disambiguation

Sequence of faces of a polytope Wikidata

A sequence of faces of a given polytope, one of each dimension up to that of the polytope (formally, though in practice not always explicitly, including the null face and the polytope itself), such that each face in the sequence is part of the next-higher dimension face. Wiktionary