Game concept
Era
The game also links it as Eras.
From the game's encyclopedia
Innovations can only be discovered if the Culture has reached the requisite Era. The next Era can only start past a certain year, and if at least 50% of the Innovations of the previous Era have been discovered.
Rulers with Tribal Government and Nomadic Government can only make use of Tribal Era Innovations.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Era is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Innovations can only be discovered if the Culture has reached the requisite Era." The game also links it as Eras. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.