The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fenestra
    n 1: a small opening covered with membrane (especially one in
         the bone between the middle and inner ear)

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "fenestra":
   aperture, broaching, cavity, chasm, check, clearing, cleft, crack,
   disclosure, fistula, fontanel, foramen, gap, gape, gat, gulf,
   hiatus, hole, hollow, inlet, interval, lacuna, laying open, leak,
   opening, opening up, orifice, outlet, passageway, pore, slot,
   space, split, stoma, throwing open, uncorking, unstopping, yawn