In a musical context, "Classical" is a broad term, but it usually refers to music that has its roots in the traditions of Western liturgical and secular music, it covers a broad period of time, anywhere from roughly the 9th century to current times. Sometimes, "Classical" is also blended up with Opera and would generally be the backbone to most music behind any Opera.
The most common time for Classical styles was between the mid 1500s and the 1900s, this period became known as the common practice period.
European classical music is largely distinguished from many other sub genres of Classical music by its system of "staff notation", which was taken on around the 16th century. Western staff notation is used by composers to prescribe to the performer the pitch, tempo, individual rhythms and exact execution of a piece of music. This leaves little room for musicians to employ improvisation and ad lib techniques.
Non European classical
Non Europeen classical can be heard all over the world and these styles of Classical are a lot more open to improvisation and ad lib techniques than the European style, some examples are below
Indian classical music
Japanese traditional music
The time period after the second world war saw classical music falling behind the commercial success of corporate-driven popular music which saw the Youth culture creating a rebellion against the “old” formula of music.
The term “classical” didn't turn up until the early days of the 19th century, this was 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 the early 1800s and states this;-
serious music following long-established principles rather than a Folk, Jazz, or popular tradition.
(more specifically) music written in the European tradition during a period lasting approximately from 1750 to 1830, when forms such as the symphony, concerto, and sonata were standardized.
Classical combining with other genres
Classical music has become combined with many other genres of music over the years, one would be Heavy Metal and thise who have a deep understanding of music will be aware of some similarities within each style, creating a style that has become known as Neo-Classical Metal.
Also, if you find a copy, check out the Electric Light Orchestra's cover of Chuck Berry's "Roll Over Beethoven", where they successfully blended Beethoven's 5th Symphony with the Rock n Roll of Chuck Berry.
(We do love that cover).