lacuna

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An empty space or a missing part; a gap: "self-centered in opinion, with curious lacunae of astounding ignorance” ( Frank Norris).
2. n. Anatomy A cavity, space, or depression, especially in a bone, containing cartilage or bone cells.

Century Dictionary and Cyclopedia

1. n. A pit or hollow. Specifically— In bot.:
2. n. A gap; a hiatus; especially, a vacancy caused by the omission, loss, or obliteration of something necessary to continuity or completeness.
3. n. In conchology, the typical genus of Lacunidæ. L. vincta is a common New England species. This small shell resembles a periwinkle, but. is thinner and of slenderer form; it is of a reddish or brownish horn-color, with two or more darker spiral bands. It is found on the sea-shore, where the animal feeds on algals.

Wiktionary

1. n. A small opening; a small pit or depression; a small blank space; a gap or vacancy; a hiatus.
2. n. An absent part, especially in a book or other piece of writing, often referring to an ancient manuscript or similar.
3. n. microscopy A space visible between cells, allowing free passage of light.
4. n. linguistics A language gap, which occurs when there is no direct translation in the target language for a lexical term found in the source language

GNU Webster's 1913

1. n. A small opening; a small pit or depression; a small blank space; a gap or vacancy; a hiatus.
2. n. (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.

WordNet 3.0

1. n. a blank gap or missing part
2. n. an ornamental sunken panel in a ceiling or dome