an extended piece for orchestra, typically in four contrasting movements
a composition of similar proportions
Now exclusively used to designate a large-scale work for full orchestra conventionally in four contrasted movements, the term symphony originally had a variety of meanings: in the baroque, for instance, it described vocal music with instruments or the orchestral introductions or interludes in an opera. Its development as a preeminent genre in its own right began in the 18th cent. and reached its classical peak in the 104 symphonies of Haydn. It was taken into wholly new territory by Beethoven, who in his nine symphonies expanded the genre which culminated in the massive, all-embracing works of Mahler in the 1900s — Andrew Clements
something of great harmonious complexity or variety
The room was a symphony in blue
chiefly NAmer = symphony orchestra
[Middle English symphonie harmony, via Old French and Latin from Greek symphōnia, from symphōnos concordant in sound, from sym- + phōnē voice, sound]