Like so many genre labels,

folk music

is a malleable term.

Does it denote roots music from regional sources, played by nonprofessional musicians? Or acoustic music with an overt political content? Or something that reclaims and archives traditional songs?

Or has the term become, as Colin Meloy fears, so vague and amorphous that it's nearly meaningless?

"Most people, when they hear the word folk music, just assume it's any sort of pop-derived music that features an acoustic guitar centrally," he says. "That's the late-20th-, early-21st-century definition of folk music."

Meloy's band, the Decemberists, may seem an odd choice for this weekend's Philadelphia Folk Festival. The Portland, Ore., group comes from the indie-rock tradition, having debuted on the regional Hush label before moving to the venerable Kill Rock Stars, then to Capitol.

The band's newest album, a continuous narrative called The Hazards of Love, alludes to early heavy-metal and progressive rock, to bands such as Black Sabbath, Jethro Tull, and Led Zeppelin. Whereas previous albums featured gently rolling tunes anchored by Meloy's acoustic guitar, this one is often heavy and loud. But this summer, the band is playing several folk festivals, including Philly's.

"At first blush, with a casual listen to the record without knowing anything about it, you would maybe view it as a strange thing for a folk festival," says Meloy from a tour stop in Louisville, Ky. "Even though it has these loud guitars on it, I think it's the folkiest record we have made to date. It owes everything to the 'folk' tradition. The entire premise, the entire narrative is built of common motifs from old folk songs."

That narrative - which the Decemberists will perform straight through tomorrow afternoon - involves ill-fated lovers and various demons, and it grew from Meloy's obsession with British folk revivalists of the '60s such as Shirley Collins, Anne Briggs, and Dick Gaughan.

"Hopefully, this is kind of the apotheosis, and I will have gotten it out of my system," Meloy laughs.

This year's Folk Festival includes its share of traditional mainstays (Tom Rush, the Del McCoury Band), but it skews to younger artists, such as Philly's lap steel/hip-hop party band Slo-Mo with Mic Wrecka, jam-band marvels the Derek Trucks Band, and the hushed, captivating Iron & Wine.

The 48-year-old festival is looking to new blood. "It is kind of adapt or die," says Meloy. "I think there's a movement happening that, thankfully, the bookers and promoters and audiences are discovering these bands that maybe they had disregarded because they were 'indie rock' or whatever. In fact, a lot of these folks are heralding the folk tradition in a lot of ways. It's cool and exciting."

And it's still folk music.

Philadelphia Folk Festival, today through Sunday at Old Poole Farm, 1323 Salford Station Rd., Schwenksville. Tickets: $54-$75. Phone: 610-287-7818; www.pfs.org/PFF.php.