Many terms are borrowed from architecture or human and animal anatomy to designate and describe parts of letters and other characters. We are even speaking of type design anatomy.

The skeleton is the center line around which every part of a glyph is built (weight, contrast, curvature, terminals, etc.). Keeping the skeleton the same across multiple styles of one typeface family is, by definition, keeping the same structure, which is one of the principal ways to maintain design consistency.