It seems to be a marriage, performed by a priest but kept secret and only written up in a secret ledger. I guess it's a marriage before God only, no witnesses.
I have just recently started this book, it is in German and German not being my first language, it takes a bit to get going with it. I will post my impression when I get through, just wondering about the author's credibility. I did read all the books written by Countess Larisch, but while reading those, I know to bring several grains of salt........