Man, it's hard to say. "Pop" is derived from the word "popular" so technically, anything popular would count. But in my mind, I consider pop music to be something that is upbeat and catchy. I'd consider Nirvana rock more than pop, although they were also pretty catchy by the time "Nevermind" came along.

Dammit, I'm not making any sense. It's hard to define pop music. The term encompasses everything from Britney to the Smiths, the Beatles to Wilco, Fergie to the Zombies.

Ow, my head hurts.

I usually define "pop" as more mainstream. But then I encounter problems with that definition because there are bands that I like that definitely are not pop but are mainstream i.e Cat Power

I think its an undefinable term

Yeah, I know what you mean Whip. I always think of what's popular as being pop but at the same time, it can obviously be something else at the same time - like being obviously rock, R&B, country or whatever. It gets confusing when people start adding 'pop' to any music genre, once it gets a little mainstream or catchier. We'll have to start pronouncing it differently depending on the definition we mean.

I don't necessarily define pop as mainstream - I just define it as that bouncy poppy sound. Here are some examples of how I distinguish between rock and pop:
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Pop: The New Pornographers, Belle and Sebastian, The Decemberists, Mates of State, Stereolab
Rock: Red Hot Chili Peppers, Audioslave, Tool, Korn
"Poppy" Rock: The White Stripes, The Black Hollies, The Bravery, Maximo Park
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Then again, I'd also classify some much more mellow stuff, like Kings of Convenience and Elliott Smith (RIP) as pop as well.

Pop is music that annoys me.

In my opinion, the term "pop" is determined by what the majority of it's listeners actually listen to on the regular. Take someone like Britney Spears. What bands did most of her fans listen to? N*SYNC? The Backstreet Boys? Christina Aguilera? Nowadays, there's Fergie. A lot of her listeners like Gwen Stefani, the Black Eyed Peas of course (once they went mainstream of course..), Nelly Furtado, or Beyonce.

That's just how I look at it though. I don't think you can put terms like "pop" into a box.

Pop music is upbeat and fun and it makes me wanna dance.

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oh really?

If it's played on played on a radio station called Kiss XXX.X, I consider it pop.

Moto. I think of it as popular music that you hear on the radio. Now I think there is Pop, Pop Rock, Pop Country and so forth that I separate them by. Basically anything you can here on the radio more than once in an hour is Pop.

I think music that is played to the masses is pop.Most music these days is trendy.I don't think Nirvana is pop.I actually think the grunge stage is when music was actually really good.There's no more Nirvana,Pearl Jam,Soundgarden,Alice in Chains,etc.Once grunge died so did the creativity in music.Speaking of Soundgarden,Chris Cornell did Audioslave and that could be considered pop.I guess my point in all this babbling is that there is no real guidelines for music genres anymore and most music could be considered pop now.