In the fenestella of this altar there is a square opening through which brandea could be placed on the tomb.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 1: Aachen-Assize] Reference

In this case the sanctuary floor was not elevated above the floor of the Basilica, and therefore the fenestella occupied the space between the floor and the table of the altar, thus forming a combination tomb and table altar.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 1: Aachen-Assize] Reference

In the sixth century this venerable sanctuary was still visited, and through its fenestella the bones of the martyrs scattered on the ground within could still be seen (Marucchi, op. cit.    From Wordnik.com.  [The Catholic Encyclopedia, Volume 3: Brownson-Clairvaux] Reference

(Lat. from piscis, a fish, fish-pond, pool or basin, called also sacrarium, thalassicon, or fenestella).    From Wordnik.com.  [The Catholic Encyclopedia, Volume 12: Philip II-Reuss] Reference