The accretion of fads and fluff that has built up over the millennia - ever changing in details, ever constant in fundamentals. Although most pop-culture is S<1, there is also a rich tradition of transingularity pop-culture.
In the civilized galaxy, pop culture derives from pre-info age, Information Era, Interplanetary, Federation, Expansion, Secfed, ComEmp Era and Post-ComEmp periods. Pop culture has a tremendous impact on the contemporary society of the time, and is essential for understanding history. To give only a very few examples: a great many spacecraft were named Certe during the Expansion Era based on the tagline in the popular "neoromans in space" virchshow Antonius. At least ten planets have been named for the stages of metaneko evolution in the ComEmp popular alife pet/game metanekoari. The inhabitants of Ackazarpses still use the character of the clever handmaiden helping out her senile AI (from the music cycle by Zophernes of Klesh) as their icon. And so on...
In general the influence of the pop culture increases the more contemporary it is. Hence the retro fad in Coronese Pet Books that was so big in 10280 is rather more important on current culture than the influence of the Interactive Cryptoanime-epic Surfers on the Sagittarius A* Supermassive Wormhole Gamma-ray Jet which was all the rage during the early Central Alliance period, and that in turn has more of an impact than the late ComEmp shortlived dramaworld franchise Orbital Keep despite the fact that the latter has a loyal following even to this day.