BSS Hentai
Stands for "Boku ga Saki ni Suki datta noni," or "but I liked them first." The main character has their (typically one-sided) crush stolen by someone else. The difference between this and NTR is that there's no relationship between the MC and the "partner" prior to being "stolen," and the focus is on regret for not acting sooner. If you've ever thought a chapter should be tagged NTR but isn't, it's probably BSS.
