hmmm... well, any european music for the past 1000 years that wasn't folk music, that was written down (even if its essence was improvisational), and that has become part of a particular canon and particular evolutionary sequence. it is rather vague, but it is the best way i can think of uniting everything from hildegard up to now.
of course, in the 20th century things get crazier... now you're classical because of the lineage/evolution you're continuing, your academic background, generally your acceptance by the contemporary classical world. there are more variables now, it would seem.
George, how do you go about acquiring music from the Renaissance and earlier? And are there any good composers/pieces that serve as a good introduction?
there are plenty of great cds out there!
i won't claim to be an expert on this stuff, because i'm myself relatively new to it... but some of the central figures are Hildegard von Bingen; Leonin and Perotin (representing the Notre Dame School); Guillaume de Machaut, Landini, Guillaume Dufay, Josquin Desprez, Orlande de Lassus, Giovanni da Palestrina, Giovanni Gabrieli, and the Englishmen Tallis and Byrd (also Dunstaple, Power, and Gibbons - Glenn Gould's favorite composer)
through these you get to witness the evolution of harmony, rhythm, and form in the european classical tradition, how from the early primitive chants we end up with bach, and even schoenberg and cage... (of course there are other developments and variables besides these composers - evolution of instruments, of tuning systems, of notation, of art patronage, etc)