An Attainder refers to the expulsion of a Bajoran from the Bajoran faith. It is similar to the ancient human practice of excommunication. A Bajoran who has been Attainted was forbidden to enter any Bajoran temple, read any prophecies, look into an Orb, pray with other Bajorans, or even to wear their earring.
In 2346, Kai Arin publicly renounced Opaka Sulan's status as a Vedek, but did not have her Attainted, for speaking out against the traditional system of d'jarras and calling for reform and unity among all Bajorans. Still, Opaka was subsequently forced to leave the shed in which she had been living with her teenage son Fasil and travel the streets to live off of the charity of others. (TLE - Terok Nor novel: Night of the Wolves)
In 2376, the Vedek Assembly had Kira Nerys Attainted as she had defied them by uploading a book of prophecies on to the Bajoran comnet. This book, which had been recovered from B'hala, was controversial because it described the Prophets as teachers rather than gods, and the Vedek Assembly wished the book suppressed. Eventually, former Kai Opaka convinced the vedeks to welcome Kira back into the faith. (DS9 - Avatar novel: Book Two; DS9 novel: Unity)