Source : Webster's Revised Unabridged Dictionary (1913)

Fenestra \Fe*nes"tra\, n.; pl. {Fenestr[ae]}. [L., a window.]
   (Anat.)
   A small opening; esp., one of the apertures, closed by
   membranes, between the tympanum and internal ear.

Source : WordNet®

fenestra
     n : a small membrane-covered opening (especially one in the bone
         between the middle and inner ear)
     [also: {fenestrae} (pl)]