As a young-er person in the West, I'm well aware of the term's general meaning. The linked article you provide fails to define it in any way that would be empirically meaningful with regard to the study of birth rates. You haven't given us any reason to suspect that American college students, or their European peers, engaging in casual sex (the focus of the linked article) has any impact on birth rates.
Based on the above, as well as your lack of engagement with any of the fertility-related factors I provided, I'm assuming you're more interested in voicing some sort of cultural critique than discussing actual drivers behind trends in birth rates.
It's pretty well defined and has been for some time. The link was posted in the thread, but here you go https://en.wikipedia.org/wiki/Hookup_culture.