The Collaborative International Dictionary of English v.0.48:

German \Ger"man\, n.; pl. Germans[L. Germanus, prob. of Celtis
   origin.]
   1. A native or one of the people of Germany.
      [1913 Webster]

   2. The German language.
      [1913 Webster]

   3.
      (a) A round dance, often with a waltz movement, abounding
          in capriciosly involved figures.
      (b) A social party at which the german is danced.
          [1913 Webster]

   High German, the Teutonic dialect of Upper or Southern
      Germany, -- comprising Old High German, used from the 8th
      to the 11th century; Middle H. G., from the 12th to the
      15th century; and Modern or New H. G., the language of
      Luther's Bible version and of modern German literature.
      The dialects of Central Germany, the basis of the modern
      literary language, are often called Middle German, and the
      Southern German dialects Upper German; but High German is
      also used to cover both groups.

   Low German, the language of Northern Germany and the
      Netherlands, -- including Friesic; Anglo-Saxon or
      Saxon; Old Saxon; Dutch or Low Dutch, with its
      dialect, Flemish; and Plattdeutsch (called also Low
      German), spoken in many dialects.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Plattdeutsch \Platt"deutsch`\, n.
   The modern dialects spoken in the north of Germany, taken
   collectively; modern Low German. See Low German, under
   German.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Plattdeutsch
    n 1: a German dialect spoken in northern Germany [syn: Low
         German, Plattdeutsch]