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
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
sonata [I], sonate [pl], suonato—Originally, "sounded" rather than "sung" (sonar vs. cantar), i.e. instrumental music. According to historical period, sonata began to imply a formal plan of movements as well as the structure within a single movement, e.g. "sonata form" and applies to instrumental sonatas, string quartets, symphonies, etc., all examples of the sonata principle. In general usage as a specific work title (e.g. violin sonata), it designates a multi-movement piece for solo or duo instruments with one of the instruments enjoying a feature role. wiki
string quartet, Streichquartett [G], quatuor à cordes [F], quartetto d'archi [I], quartetto di cordi [I], cuarteto de cuerda [S], vonósnégyes [H]—ensemble or a work written for that ensemble comprising 2 violins, viola, and cello. This is one of the essential genres/forms/ensembles of chamber music and arguably a critical "core" of "classical" music. Explore the string quartet | wiki