hiatus

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gap or interruption in space, time, or continuity; a break.
• noun Linguistics A slight pause that occurs when two immediately adjacent vowels in consecutive syllables are pronounced, as in reality and naive.
• noun Anatomy A separation, aperture, fissure, or short passage in an organ or body part.

from The Century Dictionary.

• noun An opening; an aperture; a gap; a chasm.
• noun In anatomy, a foramen.
• noun In grammar and prosody, the coming together of two vowels without intervening consonant in successive words or syllables of one word.
• noun A space from which something requisite to completeness is absent, as a missing link in a genealogy, an interval of unknown history, a lost or erased part of a manuscript, etc.; a lacuna; a break.
• noun Specifically, in logic, a fault of demonstration, consisting in the omission to prove some premise made use of, and not self-evident or admitted.

from the GNU version of the Collaborative International Dictionary of English.

• noun An opening; an aperture; a gap; a chasm; esp., a defect in a manuscript, where some part is lost or effaced; a space where something is wanting; a break.
• noun (Gram.) The concurrence of two vowels in two successive words or syllables.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A gap in a series, making it incomplete.
• noun An interruption, break or pause.
• noun A vacation, break from work.
• noun geology A gap in geological strata.
• noun anatomy An opening in an organ.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a missing piece (as a gap in a manuscript)
• noun an interruption in the intensity or amount of something
• noun a natural opening or perforation through a bone or a membranous structure