Classical is how you look at it. To most people, classical music is whatever happens to be written before about 1900 that you hear played in orchestra halls and NPR. To classical fans, there’s more nuance involved. More ambiguity, too.
“Classical music” is generally divided into about four eras, one of which is itself confusingly called the “classical era”. They run as follows.
Baroque: From about 1600-1750. To pick a representative example of this beautiful and staggeringly diverse genre at random from playlist.com, here’s Bach’s Suite No. 3 In D – Air ‘On The G string’
Classical: 1750-1820. Guys like Mozart, writing things like this example of perfection: Symphony No. 40.
Romantic: 1820-1900. Dramatic and emotional, with some particularly noteworthy examples like Wagner’s Ride of the Valkyries.
Modern: Anything more recent than 1900. It’s all awful. Well, that’s a lie. But it is all totally nuts.
Physics is often divided into “classical physics” and “modern physics”. Where the line is varies wildly depending on who you ask. Here’s where I’ve seen the line put:
*Newton is classical, relativity and beyond is not. But then again the word “classical” is often used to mean quantum. So…
*Relativity is classical, quantum mechanics and beyond is not. But then again the entirely of regular quantum mechanics is called “semiclassical” because it quantizes the particles but not the fields. I’ve even seen the “semi” dropped on a few occasions.
*Quantum mechanics is classical, quantum field theory and beyond is not.
You could probably put the line in a few other places if you wanted. It really depends on context. Personally though if I had to set a hard and fast line I’d choose the first. Relativity was a truly paradigm-shattering concept and I think the Annus Mirabilis makes a nice bright line in the sand.