allegro [I]—fast, lively tempo. From the Italian word for cheerful or gay.
andante [I]—moderately slow tempo (e.g. walking). Faster than adagio but slower than allegretto
assai—very much. Intensifies the direction of its neighboring words, e.g. "allegro assai", very fast
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
energico, con energia, énergique [F], avec énergie [F], energisch [G]—with energy, force, vigor and power
moderato [I], moderamente, modéré [F], modérément [F]—moderately, at a moderate tempo, applying a touch of restraint to its related word(s), e.g. allegro moderato
molto [It]—very much
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
scherzo—lively, brisk, typically in a triple meter; usually a three-part form with central, contrasting trio
sextet, sextour [F], sestetto [I]—six. Ensemble (or work) for six players.
sostenuto [I], soutenu [F]—sustained, perhaps a bit slow
string sextet—ensemble / composition typically for 2 violins, 2 violas and 2 cellos
trio (2)—The middle section of three-part (ternary) forms such as the minuet and scherzo yielding the structure: Minuet - Trio - Minuet (repeat). The trio often brings contrast by change of key, tempo, mood or texture i.e. a shift in instrumental scoring.