Source : Webster's Revised Unabridged Dictionary (1913)

Perineurium \Per`i*neu"ri*um\, n. [NL., fr. Gr. ? about + ? a
   nerve.] (Anat.)
   The connective tissue sheath which surrounds a bundle of
   nerve fibers. See {Epineurium}, and {Neurilemma}.

Source : WordNet®

perineurium
     n : connective tissue that covers a bundle of nerve fibers