The Collaborative International Dictionary of English v.0.48:

Ellipsis \El*lip"sis\ ([e^]l*l[i^]p"s[i^]s), n.; pl. Ellipses
   ([e^]l*l[i^]p"s[=e]z). [L., fr. Gr. 'e`lleipsis a leaving,
   defect, fr. 'ellei`pein to leave in, fall short; 'en in +
   lei`pein to leave. See In, and Loan, and cf. Ellipse.]
   1. (Gram.) Omission; a figure of syntax, by which one or more
      words, which are obviously understood, are omitted; as,
      the virtues I admire, for, the virtues which I admire.
      [1913 Webster]

   2. (Geom.) An ellipse. [Obs.]
      [1913 Webster]

   3. (Printing) a printing symbol, usually three periods in a
      row (. . .), indicating the omission of some part of a
      text; -- used commonly in quotations, so as to suppress
      words not essential to the meaning. A long dash (---) and
      three asterisks (* * *) are sometimes used with the same
      meaning.
      [PJC]

WordNet (r) 3.0 (2006):

ellipsis
    n 1: omission or suppression of parts of words or sentences
         [syn: ellipsis, eclipsis]

Moby Thesaurus II by Grady Ward, 1.0:

30 Moby Thesaurus words for "ellipsis":
   abbreviation, abridgment, abstract, apocope, aposiopesis, clipping,
   compression, condensation, conspectus, contraction, crasis,
   curtailment, cutting, elision, epitome, foreshortening, precis,
   pruning, recap, recapitulation, reduction, retrenchment,
   shortening, summary, summation, syncope, syneresis, synopsis,
   telescoping, truncation