ad libitum [L]—at one's pleasure, at will, at one's discretion. For instrumentation, it means optional (vs. obbligato). For musical passages, it suggests a free treatment, improvisation, liberal interpretation, etc.
chamber music, Kammermusik [G], musique de chambre [F], musica da camera [I], musica cameralis [L]—"Classical Music" for a small ensemble, generally 8 or fewer players with a canonical emphasis on 3-6 players. explore
opus [L], opera[pl], Op., Opp, WoO, Op. posth—work, as in a work of art. For unique identification, publishers/composers often assign an "opus number", abbreviated as "Op." (e.g. Op. 1). A range of works by opus number, uses the plural "opp." (e.g. "Opp. 1-3"). A single opus number may refer to a collection of works (e.g. a set of quartets) giving rise to designations like "Op. 1, No. 4". For misc. works without opus numbers, "WoO" is often used (e.g."Without Opus"). Posthumously assigned numbers may use "Op. posth". Opus numbers may not precisely reflect chronology (publication vs. composition), can be erroneous, sparse or duplicative. Thus, the works of some historically important composers may use separate catalog numbers (e.g. Bach, Haydn, Schubert). See wiki
piano quartet, Klavierquartett [G], Quatuor avec piano [F]—a ensemble (work) comprising violin, viola, cello and piano. Most of the great Classical and Romantic composers produced one or more works for this combination of which many are masterworks of chamber music.
piano quintet—ensemble (work) for string quartet plus piano. One of the most grand chamber music forms. For an extra rich sound, some piano quintets omit the 2nd violin and add a bass comprising the complete four-part string section of the orchestra.
quartet, quatuor [F], quartett [G], quartetto [I], cuarteto [S]—ensemble or work for four players the most important examples being the string quartet and the piano quartet
quintet, quintette [F], quintett [G], quintetto [I]—an ensemble / work for 5 players, the typical examples being string quintet (with second viola or cello), piano quintet or woodwind quintet
serenade, serenada, serenata [I], sérénade [F]—A suite of movements like a divertimento with similar connotations of lightheartedness, pleasure and relative calm with an emphasis on lyricism and often beginning with a march. Historically evokes the evening, outdoors, an honorific or amorous tribute, especially from a suitor to a lover in a window or balcony. The word serenade has its Latin origins the Latin word for "calm" or serene. wiki
Trout Quintet, Trout Instrumentation, Piano Quintet 'Trout'—A common term for the ensemble comprising violin, viola, cello, double bass and piano, named for one of the earliest and the most famous example: Schubert's quintet nicknamed "The Trout". The term is used to distinguish it from the more standard "piano quintet" comprising string quartet and piano. For an example predating Schubert, see Hummel. explore