There are a number of different ways that you could define this.
For me, the most important aspect of a definition of popular culture is that there is a difference between "high" culture and mass culture. For example, even though Shakespeare is part of our culture, Shakespeare plays are not popular culture. A performance of The Taming of the Shrew would not be popular culture. However, the move "10 Things I Hate About You," would be popular culture even though it is based on the play.
I would say that anything that is produced for a mass audience is popular culture. This serves to distinguish works of popular culture from works that are primarily meant to be works of "serious" literature, music, or film.
This is, of course, a vague definition that has gray areas. However, that is inevitable when we talk about social sciences and the study of culture and society.