In 2019 traditional pop roared back as more superstars worked at the “speed of streaming.”

Last year, only two Billboard Hot 100 No. 1s didn’t feature a rapper. The rest were dominated by hip-hop stars like Cardi B, Post Malone, Childish Gambino, Travis Scott and, for a record 29 weeks, Drake. In 2019, while plenty of rap can still be found at the top of the chart, there’s also a long list of top 40 artists -- including Selena Gomez, the Jonas Brothers, Lady Gaga, Shawn Mendes, Camila Cabello and (in multiple appearances) Ariana Grande -- leading the way for what has been a massive comeback for pop.

“I’ve been doing this since 1983, and I watch the pop cycles ebb and flow,” says Tom Poleman, chief programming officer at iHeartMedia. Poleman compares the mainstream’s end-of-decade swing back to pop after the genre’s extended dormancy to a similar shift two decades ago, when the Backstreet Boys and the Spice Girls burst out of what had been a grunge-led ’90s. “We’re coming out of that doldrum phase,” notes Poleman, “and we’re getting back into a really good place for pop -- mainstream is cool again.”

Poleman says that he first noticed the shift in early spring, when a number of long-dormant marquee names returned to the top 40 in quick succession. “I would meet with the labels a few months before the projects were coming out, and they would say, ‘Oh, this is coming out,’ or ‘That’s coming out’ -- it was Jonas Brothers, Taylor Swift, Camila Cabello, Ed Sheeran ... one after another. I was like, ‘Oh, my God. All these big pop artists are coming at the same time.’”

Radio isn’t the only platform where pop music regained ground in 2019. At streaming services, long thought to be the province of hip-hop and dance artists, many of these pop stars were posting their best showings yet. Gomez, the JoBros and Grande all snagged No. 1s on Billboard’s Streaming Songs chart.

Apple Music head of pop programming Arjan Timmermans believes that Grande’s reemergence in late 2018 was the beginning of the mainstream’s pop rebound. “Rapidly after [her fourth studio album] Sweetener, she put out [new single] ‘Thank U, Next,’ ” he says. “She was releasing music at the speed of streaming.”

The so-called “speed of streaming” has been synonymous with the world of rap, where artists have been much more willing to release new songs at faster, less predictable rates, without lengthy promotional pushes or even accompanying albums. “Hip-hop has understood this for years now,” says Timmermans, “that it’s all about keeping fans engaged by releasing content very quickly, instead of having a six-month setup and then working a radio single for another eight months, which really didn’t help pop during the streaming age.”

The poster child for pop thriving in the modern era is 18-year-old Billie Eilish, whose first album, When We All Fall Asleep, Where Do We Go?, set streaming records after a three-year trickle of music built anticipation for her full-length debut to a fever pitch. Eight months after the album release, Eilish issued a new single, “Everything I Wanted,” and promptly scored another top 10 hit. “If you take Billie, or [K-pop group] BTS -- they’re artists that grew up in the streaming age, so they know how to release music perfectly for their audience,” says Jeremy Erlich, head of music strategy/interim co-head of music at Spotify.

To recognize Eilish as a defining pop star of 2019, however, is to also understand just how much the definition of “pop” has evolved in the last decade. Grande leaned heavily on hip-hop and R&B production on hits like “7 Rings,” while Halsey’s “Nightmare” is largely indebted to alternative rock and grunge -- yet both still play as “pop” on most radio stations and streaming playlists. Then there’s Eilish: This year’s biggest mainstream breakout artist is all but unclassifiable. “Billie is the biggest pop star in the world, but it doesn’t sound like traditional pop -- it’s definitely alternative and urban-leaning,” says Erlich. “The younger generation doesn’t really think by genre.”

By expanding the definition of the genre, acts that may have traditionally been viewed as hip-hop could now be considered pop. Timmermans cites Lizzo, Post Malone and Lil Nas X as artists who could belong to both. “Part of my job is to understand how pop is moving and how the center of pop is moving, and how we can push pop forward,” he says. “Not just promoting the things that are considered pure pop, but also the things that are the unicorns and the outliers to ensure that pop maintains its strength.”

Between the increased variety of music labeled as pop and the resurgence of some of the decade’s more traditional mainstream artists, those involved and invested in the genre’s success see only greater things in the decade to come. “It’s going to get even more pop in the 2020s,” says Poleman. “But the more variety you get in there, the better.”

This article originally appeared in the Dec. 21 issue of Billboard.