The Collaborative International Dictionary of English v.0.48:

Lacuna \La*cu"na\ (l[.a]*k[=u]"n[.a]), n.; pl. L. Lacun[ae]
   (l[.a]*k[=u]"n[ae]); E. Lacunas (l[.a]*k[=u]"n[.a]z). [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.
      [1913 Webster]

   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.
      [1913 Webster] Lacunal

WordNet (r) 3.0 (2006):

lacuna
    n 1: a blank gap or missing part [syn: lacuna, blank]
    2: an ornamental sunken panel in a ceiling or dome [syn:
       coffer, caisson, lacuna]

Moby Thesaurus II by Grady Ward, 1.0:

111 Moby Thesaurus words for "lacuna":
   alveolation, alveolus, antrum, aperture, armpit, arrearage, basin,
   bowl, breach, break, broaching, caesura, cavity, cessation, chasm,
   check, clearance, clearing, cleft, concave, concavity, crack,
   crater, crypt, cup, defalcation, defect, deficiency, deficit,
   depression, dip, disclosure, discontinuity, distance between,
   double space, em space, en space, fenestra, fissure, fistula, fold,
   follicle, fontanel, foramen, freeboard, funnel chest, gap, gape,
   gat, gulf, hair space, half space, hiatus, hole, hollow,
   hollow shell, inlet, interim, intermediate space, intermission,
   interruption, interspace, interstice, interval, jump, lack,
   laying open, leak, leap, leeway, letup, lull, margin, missing link,
   need, omission, opening, opening up, orifice, outage, outlet,
   passageway, pause, pit, pocket, pore, punch bowl, room, scoop,
   shell, shortage, single space, sink, sinus, slot, socket, space,
   space between, split, stoma, suspension, throwing open,
   time interval, trough, ullage, uncorking, unstopping, vug, want,
   wantage, yawn