I guess I would define classical music from the era it was written as well as the people who wrote it.
My personal favorite is Mozart and his Requiem is spectacular.
I do love a lot of the classics like Bethoven, Bach, Tchaicovsky but have always been partial to Mozart.
I think most people think of classical music from an era point of view and I say that because we now have "Classic Rock" which refers to the 60's and 70's era rock music.
When someone mentions classical music, I just immediately think of Mozart, Bach, etc...