This article is from the Ballet and Modern Dance FAQ, by Tom Parsons twp@panix.com with numerous contributions by others.
There are many definitions; here's one of the earliest: Ballet is
"the geometrical groupings of people dancing together, accompanied by the
varied harmony of several instruments" (Balthasar de Beaujoyeulx, writing
in 1582). This definition omits one feature commonly associated with
ballets: they tend to tell stories. (Beaujoyeulx's own ballet told a
story.) On the other hand, many modern ballets--for example, many of
Balanchine's--have no explicit plot. So we might also say, ballet is
dancing done as a theatrical performance--as an art, in fact--frequently
telling a story, and drawing on a tradition of expressive movements dating
back to Beaujoyeulx and probably earlier.
Ballet normally consists only of dancing and music. But a few
ballets have been choreographed for performance without music, and some
ballets have included singing or recitation. Beaujoyeulx's ballet called
for speeches from some of the characters, and the ballets of Jean-Philippe
Rameau (1683-1764), called "ballets" on the title-pages of their scores,
are actually opera-ballets. But normally it is expected that any story
incorporated in a ballet will be conveyed by dance and mime alone.
An answer along different lines might be that ballet is the foun-
dation of all of Western theatrical dance. People aspiring to be modern
dancers or to be dancers in show business are frequently advised to start
with ballet before specializing in these other forms. Many people in the
rec.arts.dance group also report that a grounding in ballet makes you a
better ballroom dancer.
 
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