Lipograms are defined by Webster's Third New International Dictionary as "a writing not having a certain letter" but this definition is not satisfactory. In short passages it is not uncommon for some of the less common letters of the alphabet, such as X and Z, not to appear. Also, by common convention the letter or letters that do not appear are supposed to be common letters for the passage to be considered a lipogram.
"Robin's Gazette-Free Lipogram,"
Word Ways: Vol. 30
, Article 8.
Available at: http://digitalcommons.butler.edu/wordways/vol30/iss3/8