Attainder is a royal decree made by the King of the Andals and the First Men or the Hand of the King officially denouncing a person or an entire noble House. Such a decree strips the one attainted of all lands, titles and incomes. Attaintment is sometimes accompanied with a death sentence, but that may not always be the case.
Known cases of attainder
- Ser Gregor Clegane, head of House Clegane, is formally stripped of all lands and titles by Ned Stark for pillaging the Riverlands. However, the attainder is never enacted as Ned is arrested and then beheaded for treason.
- House Tully, Lords of Riverrun and Lords Paramount of the Trident, are stripped of lands and titles by Tywin Lannister for rebellion against the Iron Throne. Riverrun, and rule of the Riverlands is granted to Lord Walder Frey, Lord of the Crossing and head of House Frey. Later, Lord Frey and every known male member of House Frey is assassinated by Arya Stark, and the Riverlands fall into anarchy. Edmure Tully then escapes captivity and reclaims his lordship over the Riverlands as a vassal to King Bran the Broken.
In the books
In the A Song of Ice and Fire series, the manner in which Eddard Stark denounces Gregor Clegane is similar, but from what little context is given in the scene it wasn't clear if "attaint" was simply being used as a synonym for "denounce" or if it signified a specific, formal action.
Following the Battle of the Blackwater, House Florent is attainted by King Joffrey, with all its lands and titles being awarded to Ser Garlan Tyrell, the second son of Lord Mace Tyrell, who so far has been cut from the series.
The prequel novella The Princess and the Queen, covering the events of the civil war known as the Dance of the Dragons which occurred 170 years before the War of the Five Kings, made it explicitly clear that attainting is indeed a specific legal action. At one point when the war had turned in favor of Rhaenyra Targaryen, she told her advisors that she wanted to attaint those Great Houses who had sided with her half-brother Aegon II Targaryen. Specifically, she wanted to attaint House Lannister and House Baratheon, and give their lands away to her own loyal supporters.
What exactly "attainting" includes isn't clear, as other Houses have had their lands seized before as punishment. Whatever the case, Rhaenyra's own advisors felt that attainting was such a shockingly brutal move that they pleaded with Rhaenyra not to do it, saying that it would set a bad precedent and discomfort her current allies.