European classical music is largely distinguished from many other non-European and popular musical forms by its system of staff notation, in use since about the 16th century. Western staff notation is used by composers to prescribe to the performer the pitch, speed, meter, individual rhythms and exact execution of a piece of music. This leaves less room for practices, such as improvisation and ad libitum ornamentation, that are frequently heard in non-European art music (compare Indian classical music and Japanese traditional music), and popular music.
The period after WWII saw classical music falling behind the commercial success* of corporate-driven popular music which subsumed an image of youthful (US) rebellion against "old" (European) formalisms.
The term "classical music" did not appear until the early 19th century, in an attempt to "canonize" the period from Johann Sebastian Bach to Beethoven as a golden age. The earliest reference to "classical music" recorded by the Oxford English Dictionary is from about 1836.
* The number of CDs sold is not indicative of the popularity of classical music classical contemporary classical modern classical neo-classical classical guitar classical music neo-classical metal classical piano classical crossover 20th century classical indian classical neo classical romantic classical post-classical classical pop classical rock dark classical russian classical classical metal classical composer 20th century classical composer 21st century classical czech classical non-classical classical pianist classical jazz classical period persian classical turkish classical hindustani classical