Tortoise will perform at the Auditorium Theatre accompanied by the Chicago Philharmonic after releasing its first new album in nine years.

Courtesy of Heather Cantrell

For decades, the instrumental band Tortoise was the face of the genre known as “post rock.” The catch-all term emerged in the late 1990s to describe music that existed in the gaps between rock and jazz and between compositional music and straightforward improvisation.

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But to Tortoise co-founder John Herndon, the term “post rock” — despite his band birthing the category — never made a lot of sense.

“When I first heard it, my kneejerk reaction was, ‘I never got rid of my AC/DC records, I don’t know what this means.’ I didn’t throw my Ramones records away. I still love Chuck Berry,” he said.

His befuddlement reflects the impossible task of trying to define Tortoise, a quintet rooted in Chicago that features some of the city’s most adventuresome players — percussionists Herndon and Dan Bitney, drummer John McEntire, guitarist Jeff Parker and bassist Doug McCombs. The band headlines the Auditorium Theatre Tuesday and will appear with the Chicago Philharmonic (WBEZ is co-presenting the show with the Auditorium).

Despite dominating Chicago’s homegrown independent music scene in its first two decades, the band, like its namesake, is becoming more deliberate in its output. Three of the five band members have relocated from Chicago to the west coast and all are busy in other musical projects including Isotope 217, Chicago Underground, Brokeback and Eleventh Dream Day. The band’s eighth album “Touch” (International Anthem/Nonesuch), released this month, is the first Tortoise release in nine years.

For those unaccustomed to their music, “Touch” offers a good entry point. Opposed to the chilled-out soundscape music of former records, the melodies of these new songs sit at the forefront. “Vexations,” the album opener, is the first fully rock moment — driven by guitar-drum swagger, it builds into a fully synthesized groove. Programmed beats interplay on “Oganesson” until half-way when Parker interjects by layering in jazz chords. Other hybrid songs sync digital tools with human instruments, and on “Layered Presences,” even some field recordings. The most cinematic song is album ender “Night Gang,” which pulls in elements of spaghetti western soundtracks and cresting synthesizers.

“When Tortoise is operating best, we can set a mood and fill a space with a unique sound that is not really jazz, not really rock, not really electronic. But if people approach it with an open mind and let it be what it is, then it’s the best,” band co-founder John Herndon said.

Courtesy of Yusuke Nagata

Tortoise benefited from the collaborative vibe in the city at the time. All five members came from disparate corners of the music world, having served in bands as different as Eleventh Dream Day, Slint, and the Poster Children.

“People were experimenting with these new ways to approach the music,” he said. “I think if there is a definition to post rock, I think that’s kind of it. Rather than being about a specific sound, it’s more of a space to present ideas.”

As Tortoise’s popularity grew, the band began accepting invitations to play jam band festivals in the U.S. and Europe.

“We opened up for some bigger jam bands at some point and we got booed a few times on the stage playing in front of that crowd. I don’t know if we are a ‘make people dance’ band, which it seems like a lot of bands in that scene wanted to do, which was to have these extended improvised dance spaces. We weren’t really doing that,” he said.

As always, Tortoise has survived by doing its own thing.

“Tortoise falls between the cracks between a lot of stuff,” Herndon said. “When Tortoise is operating best, we can set a mood and fill a space with a unique sound that is not really jazz, not really rock, not really electronic. But if people approach it with an open mind and let it be what it is, then it’s the best.”