What Does It Mean?

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From our dictionary:

(lu?kyOO'nu), n., pl. -nae (-nE), -nas.
1. a gap or missing part, as in a manuscript, series, or logical argument; hiatus.
2. Anat. one of the numerous minute cavities in the substance of bone, supposed to contain nucleate cells.
3. Bot. an air space in the cellular tissue of plants.