Source : Webster's Revised Unabridged Dictionary (1913)

Lacunar \La*cu"nar\, n.; pl. E. {Lacunars}, L. {Lacunaria}. [L.]
   (Arch.)
   (a) The ceiling or under surface of any part, especially when
       it consists of compartments, sunk or hollowed without
       spaces or bands between the panels. --Gwilt
   (b) One of the sunken panels in such a ceiling.

Source : WordNet®

lacunaria
     See {lacunar}