Pop music is an abbreviation of "popular music". What is considered "pop music" has been changing constantly since the conception of music. In short, the most popular and widely accepted music of a period of time (century, decade, year, etc) can be considered "pop music".
Pop music has been the most popular music since the 1950s. Since technology today has advanced, pop music is now more techno than it was back then. Pop music is the most popular music followed by Rock and Roll.