The Collaborative International Dictionary of English v.0.48:
Endoneurium \En`do*neu"ri*um\, n. [NL., fr. Gr. 'e`ndon within + ? a sinew, nerve.] (Anat.) The delicate bands of connective tissue among nerve fibers. [1913 Webster]WordNet (r) 3.0 (2006):
endoneurium n 1: delicate connective tissue around individual nerve fibers in nerve