The Collaborative International Dictionary of English v.0.48:

Chronogram \Chron"o*gram\, n. [Gr. ? time + ? writing,
   character: cf. F. chronogramme.]
   1. An inscription in which certain numeral letters, made to
      appear specially conspicuous, on being added together,
      express a particular date or epoch, as in the motto of a
      medal struck by Gustavus Adolphus in 1632: ChrIstVs DVX;
      ergo trIVMphVs. - the capitals of which give, when added
      as numerals, the sum 1632.
      [1913 Webster]

   2. The record or inscription made by a chronograph.
      [1913 Webster] Chronogrammatic