The Collaborative International Dictionary of English v.0.48:

Fenes-tella \Fen`es-tel"la\, n. [L., dim. of fenestra ? window.]
   (Arch.)
   Any small windowlike opening or recess, esp. one to show the
   relics within an altar, or the like.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fenestella
    n 1: oval or circular opening; to allow light into a dome or
         vault [syn: lunette, fenestella]