What is "lacunars"

Lacunar \La*cu"nar\, n.; pl. E. Lacunars (l[.a]*k[=u]"n[~e]rz), L. Lacunaria (l[a^]k`[-u]*n[=a]"r[i^]*[.a]). [L.] (Arch.)

1. 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

2. One of the sunken panels in such a ceiling.

n. (plural of lacunar English)