The Collaborative International Dictionary of English v.0.48:

Yiddish \Yid"dish\, n. [G. j["u]disch, prop., Jewish, fr. Jude
   Jew. See Jew, Jewish.]
   A language used by German and other Jews, being a Middle
   German dialect developed under Hebrew and Slavic influence.
   It is written in Hebrew characters.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Yiddish
    n 1: a dialect of High German including some Hebrew and other
         words; spoken in Europe as a vernacular by many Jews;
         written in the Hebrew script