The funny thing is, klezmer hasn't been around very long. As a term describing a musical genre, klezmer is a relatively recent coinage, only dating back to about 1980. At the beginning of renewed interest in Old World Jewish wedding music and immigration-era dance tunes, a name was needed, and klezmer — the Yiddish term for wedding musician — stuck, as Mark Slobin points out in the introduction to "American Klezmer: Its Roots and Offshoots," a collection of essays for the most part written for the first Klezmer Research Conference at Wesleyan University in 1996 and later published in the journal Judaism.

Although klezmer is rooted in the wedding music of Eastern European Jews, it is best thought of as "an American form with distant European origins," Slobin writes. To equate klezmer with "Old World" music or "Eastern European" music is to ignore 100 years of recordings and the evolution of an oral folk music into a professionalized popular music heavily influenced by non-Jewish American music.

This isn't only a contemporary phenomenon. The earliest recordings of American Yiddish bands show their obvious debt to John Philip Sousa-style marching band music. By the 1920s, klezmer musicians were actively engaged in a dialogue with jazz — and vice-versa. The Original Dixieland Jazz Band inserted a bridge of "Ma Yofus" — the very DNA of old-time Yiddish music — into the hit tune "Palesteena," and Jewish jazz musicians were deftly incorporating Yiddish melodies into swing-band tunes by the 1930s, as Ziggy Elman did when he helped Benny Goodman transform Abe Schwartz's "Der Shtiller Bulgar" into "And the Angels Sing," featuring vocals by Martha Tilton and English lyrics by Johnny Mercer.

This is why some aficionados of contemporary klezmer have little sympathy for complaints that the music made by the more progressive, modern bands like David Krakauer's Klezmer Madness and the Klezmatics "isn't really klezmer." What really irks these complainants is that they aren't hearing Yiddish music as they remember it. But what they remember is music of a very particular time and place — typically, Yiddish music the way it was played in the mid-20th century, the heyday of Yiddish swing. As Slobin writes, "First regarded as the 'authentic roots' of klezmer, this canon increasingly looks like offshoots of a musical grafting process that took place principally in New York, headquarters of the recording industry and the Jewish publishing houses."

Looked at another way, in his exhaustively researched new history of the music, "The Book of Klezmer," musician Yale Strom points out that klezmer, like the Yiddish language itself, has been a constantly evolving form. It would be no more appropriate to consign an arbitrary historical cutoff point to klezmer, after which the music is not allowed to change, than it would be to say that Yiddish lost its authentic roots in Middle High German when it incorporated new vocabularies from Romance and Slavic languages as it traveled eastward. "Yiddish has not stayed static," writes Strom, "and neither has klezmer."

Other essays in Slobin's collection describe this process of musical maturation from different angles. These include Walter Zev Feldman's "Bulgareasca/Bulgarish/Bulgar: The Transformation of a Klezmer Dance Genre," which describes how Old World klezmers were already incorporating non-Jewish songs and styles into their repertoire and how the Eastern European dance genre slowly morphed into an American style. And in "Sounds of Sensibility," New York University's Barbara Kirshenblatt-Gimblett questions the whole notion of a klezmer revival to begin with.

These essays help undermine the basic premise of those who complain that in his latest album, "The Twelve Tribes" (Label Bleu), Krakauer crosses some fundamental line demarcating authentic klezmer. These misguided critics are disturbed by Krakauer's controversial innovations, including "Television Frailachs," which mixes TV themes into a bulgar, and some Jimi Hendrix-like electric guitar on the album's opener, "Tribe Number Thirteen." In truth, these critics are historical revisionists — longing for a pure form of klezmer that never actually existed.

In his efforts to make klezmer speak the language and accent of his time, Krakauer is arguably the most direct heir to the legacy left by the legendary 1920s clarinetists Dave Tarras and Naftule Brandwein, who played music entirely unlike that of their fathers and grandfathers back in the shtetl. Certainly Krakauer is more representative of a continuously evolving klezmer tradition than those who are dutifully engaged in klezmer neo-traditionalism, attempting to re-create the sound of 19th-century ensembles — a sound for which at this point no one currently alive can possibly be literally nostalgic.

Likewise trumpeters Frank London and Paul Brody — the former a founding member of the Klezmatics, the latter an American expatriate living in Berlin — continue to find new resonances in old music by doing what klezmer musicians have always done: listen to other music and play a new kind of klezmer dusted with co-territorial and cosmopolitan influences. In London's case, on "Brotherhood of Brass," his Klezmer Brass Allstars play alongside Gypsy and Egyptian brass bands and find common ground in the joy of playing klezmer melodies on metal instruments as fast and as loud as possible. With his Tango Toy ensemble, Brody draws upon his unique musical identity and background, which includes equal parts Duke Ellington, Charles Mingus, Gustav Mahler and Turkish dance music, in a fusion that lends new relevance to the old misunderstanding of klezmer as "Jewish jazz."

The question in the end, and the one that is most often put to me when I lecture around the country about klezmer, is: Where is the music headed? How far can it continue to evolve — by incorporating funk and rock, as with Krakauer's latest effort, or by going for more of an international folk-pop feel, as on the Klezmatics' upcoming album, "Rise Up! Shteyt Oyf!," on which synthesizers, children's choruses and vocal harmonies vie for primacy with Old World touches like violin and tsimbl? Can klezmer survive being mixed with old-time American music, as clarinetist Margot Leverett does with the Klezmer Mountain Boys (imagine "O Brother, Where Art Thou?" in Yiddish), before it loses its essence, its "klezmerness," its yidishkayt?

Or, as Strom writes:

Traditionalists will say that what [avant-gardist John] Zorn plays has no connection to the klezmer [neo-traditionalist Zev] Feldman plays. If this statement is true, then what Feldman plays bears little to no relationship to the music played by the itinerant Jewish musician who lived in the Alsace-Lorraine region in the fifteenth century.

For Strom, the key to authenticity, such as it is, can be found in the work of what he terms the "bale kulturniks" — "masters of culture" who, like their analogous revenants to Jewish religion, ba'al teshvahs, return to Jewish culture and then transmit it to others. He quotes clarinetist-composer Marty Ehrlich as saying, "you can't know what is of the moment without constantly flittering yourself through things that have historical meaning."

Slobin offers a related formula, positing klezmer as a type of "penumbra" music, whereby

most patrons of klezmer and many musicians listen and move to the melodies with a sense that they are not just here and now, but somewhere else in time.... There is a sensibility around a notion of reflected cultural light coming to us from a shadowed world.

Perhaps then the ultimate test of klezmer that pushes the envelope is how long and brightly that light shines.

Seth Rogovoy is the author of "The Essential Klezmer: A Music Lover's Guide to Jewish Roots and Soul Music."