The title is pretty much the thread.
Public poll, as always. No BS “it’s both” answers - I don’t roll like that. YOU MUST CHOOSE ONE!
Oh, and feel free to discuss. 
I think it was supposed to be a dismissive term coming from music snobs, so it kind of was both back then. Now I think you can say it’s a style, in that it’s deliberately aimed at Top 40 radio play.
Pop is the mainstream, derivative pap drawn from the popular genres. So you can have country pop, rock pop, hip hop pop etc. etc. It doesn’t stand alone as a musical form.
Forced to choose one, it’s any music that is popular. To me, the definition encompasses a number of things, but I wouldn’t pin it down to a particular genre, or “style of music” as you put it.
I don’t know jack about music, but to me it means whatever music is currently most popular with teenagers and 20-somethings, which varies in style over time. In the late 60s, the Monterey Pop Festival included Hendrix, The Who, and Joplin. In the 1955 movie Blackboard Jungle, one of the teenage characters says (paraphrasing from memory) “Put on some pop. Put on Frank Sinatra.”
Either that, or it’s music liked by dads.
Consider the fights that have broken out right here over whether the Beatles were pop, rock, or defied categorization.
Maybe more a broad category than a single style.
But there can be a pop song that only a few people have ever heard; and there can be a popular song or piece of music that would not be classified as “pop.”
Popular music, no particular style. There was even a time when jazz was popular, believe it or not.
I think pop isn’t so much a style as an approach. It’s normally vocal-centric to the point that that the instrumentation barely gets noticed, although the actual musicians themselves are highly competent. They’re not there to play flashy solos, but rather only to support the vocals. There’s a reason American Idol is about singing…singers are what’s popular.
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Style. Corporate trash.
Claiming that pop equals whatever is / was popular is preposterous. Mozart was wildly popular in his day, so his music is pop music? Music schools don’t place 1920’s Jazz into Pop studies, either, although the relativity tangent will surely get a mention.
Spectre of Pithecanthropus makes a good point.
Bubblegum rock from the 60s and 70s is rightly called “pop” even though it isn’t popular any more. “Pop” implies a certain lowest common denominator for when it was made and released. It isn’t necessarily devoid of merit, it just isn’t very ambitious or groundbreaking.
If “Good Vibrations” and “A Day in the Life” aren’t pop, what are they?
Well, they’re really only ambitious or groundbreaking if you consider them in the world of pop. Music Concrete was already a couple of decades old.
Rap music may be popular now, but it is not pop. Folk music and show tunes are popular with the people I hang out with, but it is not always pop.
Pop is a style of light music in my mind.
The answer isn’t both, it’s neither. Or maybe it’s both, but the OP says that’s BS. I would say it’s more what’s popular than it is a specific style because what Pop is has changed dramatically over the years. But maybe there is something that connects it all. Right now, most Pop is either hip hop or hip hop influenced. But not all hip hop is pop. Even popular hip hop. Migosis super popular, but not pop. Cardi B is super legit hip hop, but absolutely pop. Why? Mainstream success. Then there are people who are mixing what was pop 5 years ago with hip hop. See Ariana Grande. But there are also people doing it who are not pop. SeeLizzo, Good as Hell. Why not Lizzo? Not popular enough. But clearly in the pop style. But hip hop not pop. Or maybe it is pop? My thesis isn’t super clear, I know. This is why you have the poll right?
At the start of the decade, though, that’s not what pop sounded like. It sounded likeKaty Perryor Lady Gaga, orMaroon 5, until Lordecame along and pointed out that the lifestyle of that pop music was BS and so pop started to sound like her (sad and slow but catchy with hip hop production) and Halseyand others for a while before sliding back to being Hip Hop.
The again in the early 2000s pop wasJustin Timberlake, the Black Eyed Peas, and Matchbox 20…but it was also50 Centand Eminemwho were indisputably releasing pop hits with In Da Club and Lose Yourself even though, again, hip hop. And they weren’t, for the most part, pop artists. This was a blip for them. Go back the the late 90s and you haveThird Eye Blind, NSync, Usherand Shania Twain. Early 90s Whitney Houston, Maria Carey, BoyzIIMen, and Janet Jackson were pop. (no links I expect everyone knows what they sound like)
We can keep going backwards to when Swing was pop. When Verdi was pop. Yes, to when Hayden and Mozart were pop. If you go in 5 year jumps like that the connecting line is pretty clear. But take a look at say, Whitney Houston and Cardi B (or even the new Taylor Swift song) and you might have a harder time making the connection if you played them back to back. Clearly they are not the same stlye of music, but they come from the same musical movement and there is a very logical progression from one to the next because, what the public likes doesn’t change all that dramatically at least not for long. They want songs with catchy hooks that they can hum or sing along to. They want the songs to be short and easy to digest. They want them to be accessible and sound like other songs they already know and like. They need them to be everywhere so that they get stuck in the back of your mind. True for Motzart eine kleine nachtmusik, for Verdi arias, for Benny Goodman and Count Bassie, for the early Beatles songs (I Want to Hold Your Hand), all the way up to Memememe or whatever the heck that new Taylor Swift song is called.
Too late for the edit, the Lizzo song I linked is probably more Neo Soul than hip hop, but it’s not the most representative song of hers necessarily. She’s like Anderson Paak and Frank Ocean this way, blending hip hop and neo soul in a way that, if it was slightly more mainstream, would absolutely be pop… But isn’t.
It’s BOTH! And I’m ready to die on that hill. 
Btw, just because the poll is limited to two choices doesn’t mean the discussion forces one to take a binary position.