Excerpt from article below. Thoughts? Classical music has blurred the definition and boundaries a bit more recently (including pop instruments and rhythms), but I think it is just as rigorous as it was in the 18th century.
So today, classical music is thought to begin at that nebulous point in the early Renaissance when choral composers first appear as identifiable names. It ends...well, it doesn’t end as such, but for contemporary music to be classical it must be seen to conform to at least some of the characteristics of classical music defined above. So, for as long as composers are writing music down, using orchestral instruments, giving generic titles associated with the 18th century, or consciously absenting themselves from the performance stage, the characteristics of classical music will continue to define a specific tradition of musical activity.