What is Classical Music?

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How we define the use of the word classical in music. The word 'classical' applies to music created around the 50 years at the end of the 18th and beginning of the 19th centuries, such as that created by Haydn, Mozart and Beethoven. But the definition of classical has ended up covering a multitude of music from different eras. So what rules must music follow to be defined as 'classical'?