While originally it was defined solely as merely meaning popular music, the term has evolved, with a second definition.
This newer use defines mostly mainstream music. Catchy, simpler rhythms, repeating chorus, minimal (or no) guitar, heavy drum beats, streamlined lyrics that don't really have a deeper meaning, and so on. Acts such as
Britney Spears, Lily Allen, late 80's/early 80's-on
Michael Jackson,
Christina Aguilera, and the like... would not be categorized as rock, hip hop, R&B (while a common element of pop music), metal, blues, or anything else... it would be categorized as "Pop Music". It is a genre as well as a term referring to music that is popular.