1) A distinct elongated structure, characteristically 10 nm in diameter, that occurs in the cytoplasm of higher eukaryotic cells. Intermediate filaments form a fibrous system, composed of chemically heterogeneous subunits and involved in mechanically integrating the various components of the cytoplasmic space. Intermediate filaments may be divided into five chemically distinct classes: keratin filaments of mammalian epithelial cells; desmin filaments found in cells of muscles of all types; vimentin filaments found in mesenchymal cells; neurofilaments of neurons; and glial filaments found in all types of glial cells.

2) Any of a class of usually insoluble cellular protein fibers (as a neurofilament or an epithelial-cell cytoplasmic filament of keratin) composed of various fibrous polypeptides that serve especially to provide structural stability and strength to the cytoskeleton and are intermediate in diameter between microfilaments and microtubules.