The inner tissue or core of a rachis, which is the central shaft or stem of a feather or leaf.
From Greek 'endo-' (within) + 'rachis' (backbone, stem). The word entered anatomical and botanical terminology in the 19th century to describe the internal structure of elongated plant and animal structures.
The endorachis is why feathers are surprisingly complex—the shaft you see is just the outside, but inside there's an entire structure that's hollow and lightweight, which is exactly why birds can fly so efficiently.
Complete word intelligence in one call. Free tier — 50 lookups/day.