Great Council

A Great Council is a council formed among the lords and ladies of the Seven Kingdoms of Westeros to determine the King of the Andals and the First Men when the line of succession to the Iron Throne is unclear. Great Councils are called upon and dissolved once the next monarch is chosen to inherit the crown. They are similar to the Kingsmoots of the ironborn.

Season 8
After the assassination of Daenerys Targaryen, Jon Snow is taken prisoner by the Unsullied. An assembly is ultimately convened with some of the remaining high lords and ladies of Westeros gathering in the Dragonpit of the ruined King's Landing with their armies outside the city. Grey Worm brings Tyrion Lannister out to the council to determine his and Jon's fates, but Tyrion suggests that the decision cannot be left to Grey Worm or the nobles but instead to the king. Howevr, without another known Targaryen (Jon's true parentage being kept a secret), and with the Iron Throne having been melted, the line of succession is unclear, so Tyrion suggests that they, as the most powerful men and women in Westeros, choose the king or queen, which Grey Worm allows, turning the gathering into the Great Council of 305 AC. Samwell Tarly takes this a step further, suggesting that the smallfolk choose their ruler, but the nobles laugh this notion off. On Tyrion's prompting, Bran Stark is chosen as the new King of the Andals and the First Men. Tyrion declares that no longer will the crown be inherited, but instead chosen permanently by Great Councils. Sansa Stark asks her brother to allow the North to be independent, which he agrees to, turning the Seven Kingdoms into the Six Kingdoms.

In the books
In the A Song of Ice and Fire novels by George R. R. Martin, there has not been a Great Council called upon in the timeline of the books themselves, but there have been three Great Councils called upon previously, as well as numerous (but failed) attempts to convene Great Councils. Two were called upon when the inheritance of the throne was unclear under House Targaryen while the third was called on to determine the regent of the underage king.

The Great Council of 305 AC takes place in Season 8 of Game of Thrones. As Season 8 has surpassed the timeline of the books and is based on a general outline of the two forthcoming novels, it is not currently known if a Great Council will be gathered and assembled in the books.