An orchestra, since the late 1700s, has been a fairly well-defined conglomeration: strings, woodwinds, brass and percussion. Even if you don't know exactly what's on the program, you can buy a ticket for a symphonic concert with a fair general idea of what sounds you will hear.

Not so in chamber music. On one evening, the program might include nothing but woodwinds; on another, only brass or strings-occasionally, since the 1930s, there are even concerts devoted entirely to percussion. And chamber music composers have mixed these ingredients (sometimes stirring in a piano or two, a harpsichord, harp, marimba or guitar) in a wild variety of combinations-usually determined by the forces available for the original performance. The recipe for a string quartet or piano trio has been standardized since the time of Haydn-but what, exactly, constitutes a string quintet? If you listen to Mozart, it is a quartet with an extra viola (which Mozart loved to play). For Schubert or Boccherini, the extra instrument is a cello.

The result of this unstandardized proliferation is a lot of chamber music for unusual combinations: Mendelssohn's Octet for two string quartets, for example, or Bartok's Sonata for two pianos and percussion-not to mention the Brahms Horn Trio or Schumann's Andante and Variations for two pianos, two cellos and horn, which he later revised, dropping everything but the pianos. The good news about works like these is that they offer fresh, intriguing combinations of sound. The bad news is that they don't get performed very often because the necessary performers are not readily available.

Enter the Chamber Music Society of Lincoln Center, which has organized a group capable of handling anything that comes along in chamber music and demonstrated that fact at the Kennedy Center last week. The program included the Washington premiere of ''Sacred Ground'' by Keith Jarrett for flute, clarinet, cello and piano, along with a Beethoven string quartet, the Brahms Sextet in G and a Bach sonata for viola da gamba, transcribed for clarinet. The Lincoln Center players were not the first group to mobolize for this kind of versatility, but they have organized some of the most prestigious names in chamber music-people like flutist Paula Robison, violist Walter Trampler and the Emerson String Quartet. And, along with such organizations as Music from Marlboro, Washington's Theater Chamber Players and the even more versatile Millennium Ensemble, they have helped to create a nationwide boom in chamber music.

Besides their live performances, they have been doing this on records that explore joyfully the rich variety of the world's chamber music. One of their albums, devoted to the complete woodwind chamber music of Poulenc, was reviewed here earlier this year. Four others give a fair idea of the sheer quality and rich imagination in their programming.

The harp is the focal instrument on one record (Musicmasters MM 20031), recorded in concert at Alice tully Hall and featuring harpist Osian Ellis. The three works included-Handel's feather-light Harp Concerto in B-flat, Debussy's exotic Danses Sacree et Profane, and Britten's brilliant, witty Harp Suite-could hardly be more sharply contrasted, and all are performed with exemplary style and a minimum of audience noise.

In another live concert, recorded in 1974 (Musicmasters MM 20027), the guest artists are baritone Dietrich Fischer-Dieskau and the Juilliard String Quartet. The programming is splendidly fresh: five songs of Samuel Barber with piano accompaniment and seven of Hugo Wolf's Morike Lieder, with the piano part beautifully transcribed for string quartet by Claus Adam, who was then the Juilliard's cellist. Adams's arrangements cradle the voice in a gentle, deeply expressive sound-texture quite different in effect from Wolf's piano accompaniment, though Adam takes only a few small liberties with the original keyboard notes. The string sound is like that of other voices, wordless and content to keep a supporting role, engaging in a dialogue that makes the human voice stand out in dramatic relief. Fischer-Dieskau is outstanding in the Wolf songs, good but less at home in the Barber pieces. This record is also poignant reminder of what a glorious voice he had a dozen years ago.

Three works of Robert Schumann receive outstanding performances on two other Lincoln Center recordings. The Adnante and Variations mentioned above has one of its rare performances including the cellos and horn on Musicmasters MM 20007, and the effect is both strange and glorious. On the other side are his 10 Spanish Love Songs, which use four voices, solo and in various combinations, with two pianos for accompaniment. Among the voices, tenor John Aler is particularly notable for the purity of his tone and dramatic effectiveness of his interpretation, but excellent work is also done by soprano Kathleen Battle, alto D'Anna Fortunato and baritone Dominic Cossa, with effective piano performances by Charles Wadsworth and Richard Goode.

Finally, two works slightly closer to the standard repertoire receive outstanding performances on Musicmaster MM 20006. Schubert's Variations for Flute and Piano, D. 802, based on a melody from "Die Schone Mullerin," are interpreted by Paula Robison and Charles Wadsworth with a grace and pathos that evoke that whole, masterful song cycle. And Schumann's Piano Quartet in E-flat receives, from Jaime Laredo, Walter Trampler, Leslie Parnas and Richard Good, one of the most skilled, impassioned performances I have ever heard.