From the game's encyclopedia
A Decision is a special action that a Character can take at their leisure.
Each decision has its own set effects, which can range from minor Character changes to Empire-shaking impacts.
Decisions that move a Character to a specific location for a period of time are called Activities.
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.
Decision is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Decision is a special action that a Character can take at their leisure." It covers Extort Subjects Decision, Sell Titles Decision, Embrace Celibacy, Abandon Celibacy, Commission Epic and 14 more. The game also links it as Decisions and Decide. 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.