1) Historically the style of art music common in Europe during the period between the middle of the 17th century and the beginning of the 18th century. The works of Haydn and Mozart and the earlier work of Beethoven (who also played a large role in the transition from classical to romantic) are prime examples. Bach isn't classical. Tchaikovskij isn't classical. Chopin isn't classical.
2) In common language all art music made between the early 17th century to present, usually by composers with education or at least some knowledge of the art of composition. (In other words, practically all art music from the early 17th century up until now.)
There is a lot of prejudice against both of the above definitions of the term.
Classical music is exemplified by such composers as Haydn and Mozart.