Age of Enlightenment – a turning point in European culture and thought that focused on reason, equality, science, and the human experience.
Dramma Giocoso/Opera Semiseria (“playful drama” or “semi-serious opera” in Italian) – a type of Classical opera associated with Mozart, in which the story and feel are neither totally tragic nor totally comedic.
Form – the in-built structure of a piece of music, which includes traditional guidelines for things like harmonic progression as well as when, where, and how to introduce new melodies or themes.
Movement – an isolated portion of a larger piece of music, often performed after a brief pause or intermission. Different movements will usually explore different rhythms and musical styles, but all movements within the same piece are typically related in some way (harmonically, thematically, narratively, etc.).
Opera Buffa (“funny opera” in Italian) – an opera dealing with humorous subject matter, with roots in early-18th century Naples.
Opéra Comique (“comic opera” in French) – a style of French opera born in the 18th century. Originally a collection of familiar tunes retrofitted to a funny story, opéra comiques were often composed from scratch by the time the Classical period arrived, frequently with spoken dialogue between scenes.
Opera Seria/Tragédie Lyrique (“serious opera” in Italian or “lyric tragedy” in French) – Italian and French versions (respectively) of an opera based on a serious or tragic subject; more of a Baroque categorization.
Singspiel (“sing-play” in German) – an early German opera with spoken dialogue as opposed to sung-through recitative, the most famous of which is probably Mozart's Die Zauberflöte (The Magic Flute).