(noun)
an elaborate composition of music written for a full orchestra, often containing four sections, or a piece of orchestral music that serves as an introduction to a long vocal performance such as an opera;
e.g. The orchestra performed a symphony written by Beethoven.
(noun)
(American) a large group of musicians consisting of sections of string instruments, wind instruments, and percussion, or a performance given by such an orchestra;
e.g. We have tickets to see the symphony perform on Saturday.