Source : Webster's Revised Unabridged Dictionary (1913)

Lacuna \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.

Source : WordNet®

lacuna
     n 1: a blank gap or missing part [syn: {blank}]
     2: an ornamental sunken panel in a ceiling or dome [syn: {coffer},
         {caisson}]
     [also: {lacunae} (pl)]