From the game's encyclopedia
The Ethos of a Culture defines its core values, guiding principles, and general attitude of its members towards life in general.
In addition, the Ethos greatly affects the cost for a Culture Head establishing new Traditions.
The Culture Head can replace their current Ethos through Reformation.
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.
Ethos is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "The Ethos of a Culture defines its core values, guiding principles, and general attitude of its members towards life in general." It is part of Cultural Pillar. 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.