Atmosphere at the Henry Fonda Theater

“Underground hip-hop” is dead. Not to say that below-the-radar artists no longer exist but rather that the conventional definition of “underground” is more nebulous than ever before.

Take Atmosphere, one of the sub-genre’s standard-bearers, a group whose 1997 debut, “Overcast,” helped to establish the boundaries of what it means to be “underground”: releasing products on its own label (Rhymesayers), an aversion to the major-label bling-era excess, a focus on the personal over the external.

Yet more than a decade later, the Minneapolis duo of Slug and Ant can no longer be accurately described as “underground rap.” At least, not if you watch MTV, where Atmosphere was named artist of the week earlier this month. Or maybe you caught last week’s Billboard charts, where Atmosphere’s sixth studio LP, “When Life Gives You Lemons, You Paint That . . . Gold,” debuted at No. 5, moving more than 35,000 units to a die-hard fan base built up gradually over years of relentless touring.

Ultimately, it seems that in the download-ravaged hip-hop world of 2008, the indie model of slowly cultivating a fan base has become the prototype all successful rappers should follow. Because now only a handful of rappers can claim a better winning streak than Atmosphere, which has sold out every night of its West Coast tour, including two shows at the Henry Fonda Theater in Hollywood.

From the moment the first song, “Puppets,” ricocheted from the speakers Tuesday night, intensity was at a white-hot pitch. Supported by a guitarist, a keyboardist, a backup singer and Ant on turntables, Slug, né Sean Daley, sarcastically smirked, “Oh, you’ve heard this one before?” to the whoops and hollers of the adoring, mostly underage crowd. Although the song had been officially released only seven days prior, the audience rapped along to every word, evidencing a sort of cathartic release uncommon in contemporary hip-hop.

Before each of its shows on this tour, the group has been doing Atmosphere karaoke, but in a weird twist on the traditional version of the game, it hasn’t provided any of the lyrics, leaving it to the fans to remember the verses. And it’s difficult to imagine any other group, indie or otherwise, being able to get away with such a leap of faith, as Slug’s short-story-like tales of addiction, ex-girlfriends and self-doubt consistently struck a chord with the crowd.

As a frontman, the charismatic Slug has matured into one of hip-hop’s best performers, pantomiming the plots of his songs, delivering appreciative asides to the crowd, rapping with a swagger and confidence that belie his often self-deprecating lyrics.

The set list veered heavily toward Atmosphere’s newer material, but it did dip into its increasingly deep catalog, playing everything from “Guns & Cigarettes” and “Between the Lines” from 2001’s “Lucy Ford”; “Lovelife,” Shrapnel” and the title track from 2003’s “God Loves Ugly”; and “Sunshine” from its tour-only “Sad Clown Bad Summer -- Number 9.”

As the 90-minute set wound down with an acoustic guitar encore of “Guarantees” and “Not Another Day,” it was hard not to think back to the lyrics of the song that kicked off the show, with its paranoid murmurings about “barely trust[ing] them / they’re all puppets / love is nothing / scared of success.” It was hard to miss the irony: Slug might sing about being scared of success, but it hasn’t stopped him from achieving it.