Classical
Classical music is hard to pin down with one clean definition, but it is generally understood as music rooted in Western European traditions, performed by trained musicians in formal settings like concert halls, opera houses, and churches (AllMusic, n.d.-m). Its central value is faithful interpretation, meaning performers aim to play exactly what the composer wrote rather than improvise around it. That is the opposite instinct from jazz, where improvisation is the whole point. Classical treats the written score almost like a sacred text.
Its roots trace back to the chant melodies used in Medieval religious rites (AllMusic, n.d.-m). From there, music historians divide the tradition into distinct eras, each with its own dominant style: Medieval, Renaissance, Baroque, Classical, Romantic, and the 20th and 21st centuries (Dates of classical music eras, n.d.). One confusing wrinkle worth clearing up. "Classical music" as a broad umbrella term and the "Classical period" as one specific era are not the same thing. The Classical period itself only covers roughly 1750 to 1820, the age of Mozart, Haydn, and Beethoven, and sits between the Baroque and Romantic periods (Classical period (music), n.d.).
The Baroque period that came before it ran from about 1600 to 1750, and it is where opera and the concerto grosso were formalized as musical forms (Baroque music, n.d.). The Classical period that followed favored a clearer, simpler melodic style, with one main tune supported by chords underneath, rather than the dense, interweaving lines common in Baroque music (Classical period (music), n.d.). You could think of that shift like moving from a crowded group conversation to one clear voice speaking over quiet background support.
Today, classical music's modern form covers a wide range of output: opera, symphony orchestras, chamber ensembles, solo works, choral music, film scores, and avant-garde composition (AllMusic, n.d.-m). It is also worth noting that the line between classical and popular music is a fairly recent idea. Much of what is now treated as classical music was, in its own era, simply the popular music people were listening to at the time (AllMusic, n.d.-m).
Sources:
AllMusic. (n.d.-m). Classical music style overview. https://www.allmusic.com/style/classical-ma0000002521
Baroque music. (n.d.). In Wikipedia. https://en.wikipedia.org/wiki/Baroque_music
Classical period (music). (n.d.). In Wikipedia. https://en.wikipedia.org/wiki/Classical_period_(music)
Dates of classical music eras. (n.d.). In Wikipedia. https://en.wikipedia.org/wiki/Dates_of_classical_music_eras