Pop is a genre, it’s catchy beats with easy-to-remember lyrics. Pop is a WIDE genre, it has the normal pop, who are Perry, Spears etc but it has variations, Alternative/indie Pop (Marina, Lana, Foster the People, Vampire Weekend) electropop, dark pop, gothic pop, and it has some other genres mixed with pop, such like r&b, soul, rock. It doesn’t matter if it’s popular, there are unknown pop artists. But “Pop” can be known as an abreviation for “Popular”, any artist can be pop, if it’s this way, The Beatles were pop, Joan Jett as well, she was a popular artist, but in rock style. So there’s a difference between Pop as Popular and Pop as a musical genre.
(i submitted because it was too big!)