Most terms aren't necessarily defined strictly in only one way, maybe unless you're referring to its dictionary definition or unless it's a Constitutional law or something.
I propose that "Classical Music", in the Western sense, is:
1.) Music of the tradition spanning from the early Baroque up to some or most of the avant-garde, which relies heavily on a written score using certain standardized instruments.
- Why is this so? When you say "Classical" it's always about having a score and performing from it. Avant-garde composers are also usually trained in conservatories, which in turn are schools which originated from this tradition which "codified" Bach, Beethoven, Mozart, etc. as "golden age" composers; in other words, these contemporary composers are directly tied to the classical tradition despite being very different.
- It is a tradition because I think, more or less, most composers of this "tradition" studied each other's musical works (e.g. Beethoven studied Handel, Chopin Bach, Bartok Liszt, etc.) and were therefore familiar with each other's music
- By contrast, folk musicians don't necessarily "codify" their musics
- Pop musicians are also influenced by one another but they don't necessarily use a score in their performances
- Certain composers like John Williams, George Gershwin, and Scott Joplin aren't really "classical" composers because they didn't intend to join this "community" of "academic" music...err...wait...what...huh?
2.) Music studied and propagated in "conservatories", schools dedicated to systematizing, codifying, and enriching music of the above tradition. Individuals who study in this manner, are, therefore, "Classical" musicians.
Anyways, let's just describe it in simplest form:
The kind of music that has been codified centuries ago by a bunch of composers who wrote a lot of complex works for powerful people and is still being studied today by musicians who want to replicate this music.