filum terminale

noun

fi·​lum ter·​mi·​na·​le ˈfī-ləm-ˌtər-mə-ˈnā-(ˌ)lē  ˈfē-ləm-ˌter-mə-ˈnäl-ˌā 

plural fila terminalia ˈfī-lə-tər-mə-ˈnā-lē-ə  ˈfē-lə-ˌter-mə-ˈnäl-ē-ə 

: the slender threadlike prolongation of the spinal cord below the origin of the lumbar nerves : the last portion of the pia mater