Warning: the following article from Grove's Dictionary of Music is not for the faint of heart.
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Classical.

A term which, along with its related forms, ‘classic’, ‘classicism’, ‘classicistic’ etc., has been applied to a wide variety of music from different cultures. It evolved from the Latin classicus (a taxpayer, later also a writer, of the highest class) through the French classique into English ‘classical’ and German Klassik. In one of the earliest definitions (R. Cotgrave: Dictionarie of the French and English Tongues, 1611), classique is translated as ‘classical, formall, orderlie, in due or fit ranke; also, approved, authenticall, chiefe, principall’. The two parts of this definition will be retained here and glossed as (i) formal discipline, (ii) model of excellence, supplemented by (iii) that which has to do with Greek or Latin antiquity (Dictionnaire de l’Académie, 1694), and (iv) that which is opposed to ‘romantic’, the latter understood as morbid and unruly (Goethe, 1829). Of the various meanings, (ii) has had the widest currency over the longest time. In this general sense, for example, Forkel recommended J.S. Bach’s main keyboard works as ‘klassisch’ (1802, rendered in the English translation of 1820 as ‘classical’). Generic excellence accounts for the similar labelling of Josquin’s motets, Palestrina’s masses, Couperin’s suites, Corelli’s concertos, Handel’s oratorios and Schubert’s lieder – though as Finscher has observed (1966), the term is properly reserved for works in genres ample enough in scope and developmental possibilities to be susceptible of ‘classical’ fulfilment.

In the early modern era, it was more often in the first two senses enumerated above that the terms ‘classic’ and ‘classical’ were applied with regard to literature and art, with analogies to Greek and Roman culture only gradually coming to the fore. This was especially true as regards music (e.g. Scacchi, 1643; Schütz, 1648), for which no antique heritage was known to survive (see Nägeli, 1826). As Weber has shown (1992), it was in 18th-century England that ‘classical’ first came to stand for a particular canon of works in performance, distinct from other music in terms primarily of quality, but also to some extent age (the Concert of Ancient Music generally restricted offerings to pieces written more than two decades earlier). Civic ritual, religion and moral activism figured significantly in this novel construction of musical taste, converging notably in the cult of Handel. On the Continent, where canonic concert repertories were slower to develop (or were not entirely public, as with the Viennese concert series organized by Gottfried van Swieten during the 1780s and Raphael Georg Kiesewetter during the 1810s), ‘classical’ music continued up to the end of the 18th century to be understood mainly in its traditional senses – as when Constanze Mozart deemed the value of her late husband’s compositional fragments equal to that of ‘fragments of classical authors’ (letter of 1 March 1800). The composer’s biographer Niemetschek, in positing the ‘classical worth’ of Mozart’s music, had earlier written (1797, rev. 1808) that ‘The masterpieces of the Romans and Greeks please more and more through repeated reading, and as one’s taste is refined – the same is true for both expert and amateur with respect to the hearing of Mozart’s music’. For Spazier (1800), too, a classical work of music was one that ‘must gain from each [new] analysis’.