\La*cu"na\, n.; pl. L. {Lacun[ae]}; E. {Lacunas}. [L.,
ditch, pit, lake, orig., anything hollow. See {Lagoon}.]
1. A small opening; a small pit or depression; a small blank
   space; a gap or vacancy; a hiatus.
2. (Biol.) A small opening; a small depression or cavity; a
   space, as a vacant space between the cells of plants, or
   one of the spaces left among the tissues of the lower
   animals, which serve in place of vessels for the
   circulation of the body fluids, or the cavity or sac,
   usually of very small size, in a mucous membrane.