I just read the wiki page about pop music
and agree with the following definition:
According to Simon Frith pop music is produced "as a matter of enterprise not art", is "designed to appeal to everyone" and "doesn't come from any particular place or mark off any particular taste". It is "not driven by any significant ambition except profit and commercial reward ... and, in musical terms, it is essentially conservative". It is, "provided from on high (by record companies, radio programmers and concert promoters) rather than being made from below ... Pop is not a do-it-yourself music but is professionally produced and packaged"
which means for me that Pixies is not pop. Late Beatles is not pop, early Beatles is