1.  lacuna(n = noun.artifact) blank - a blank gap or missing part;
is a kind of crack, gap

2.  lacuna(n = noun.artifact) caisson, coffer - an ornamental sunken panel in a ceiling or dome;
is a kind of panel

1.  A small opening; a small pit or depression; a small blank space; a gap or vacancy; a hiatus. [1913 Webster]

2.  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]

lacuna, n. (pl. lacunae or lacunas)
1 a hiatus, blank, or gap.
2 a missing portion or empty page, esp. in an ancient MS, book, etc.
3 Anat. a cavity or depression, esp. in bone.

Derivative:
lacunal adj. lacunar adj. lacunary adj. lacunose adj.

Etymology:
L, = pool, f. lacus LAKE(1)

N  incompleteness, deficiency, short measure, shortcoming, insufficiency, imperfection, immaturity, half measures, defect, deficit, defalcation, omission, caret, shortage, interval, break, noncompletion, missing link, missing piece, missing part, gap, hole, lacuna, incomplete, imperfect, unfinished, uncompleted &c (complete), defective, deficient, wanting, lacking, failing, in default, in arrear, short of, hollow, meager, lame, halfand-half, perfunctory, sketchy, crude, mutilated, garbled, docked, lopped, truncated, in progress, in hand, going on, proceeding, incompletely, by halves, caetera desunt, caret.

N  interval, interspace, separation, break, gap, opening, hole, chasm, hiatus, caesura, interruption, interregnum, interstice, lacuna, cleft, mesh, crevice, chink, rime, creek, cranny, crack, chap, slit, fissure, scissure, rift, flaw, breach, rent, gash, cut, leak, dike, ha-ha, gorge, defile, ravine, canon, crevasse, abyss, abysm, gulf, inlet, frith, strait, gully, pass, furrow, abra, barranca, barranco, clove, gulch, notch, yawning gulf, hiatus maxime, hiatus valde deflendus, parenthesis, void, incompleteness, period, interim (time), with an interval, far between, breachy, rimose, rimulose, at intervals, longo intervallo.

N  concavity, depression, dip, hollow, hollowness, indentation, intaglio, cavity, dent, dint, dimple, follicle, pit, sinus, alveolus, lacuna, excavation, strip mine, trough, honeycomb, cup, basin, crater, punch bowl, cell, socket, valley, vale, dale, dell, dingle, combe, bottom, slade, strath, glade, grove, glen, cave, cavern, cove, grot, grotto, alcove, cul-de- sac, gully, arch, bay, excavator, sapper, miner, honeycomb (sponge), depressed, alveolate, calathiform, cup-shaped, dishing, favaginous, faveolate, favose, scyphiform, scyphose, concave, hollow, stove in, retiring, retreating, cavernous, porous, infundibul, infundibular, infundibuliform, funnel shaped, bell shaped, campaniform, capsular, vaulted, arched.