Game concept
Struggle
The game also links it as Struggles, Struggle's.
From the game's encyclopedia
Struggles are long-term conflicts involving large Regions with many Faiths and Cultures. Within the boundaries of a Struggle, peculiar special rules and Modifiers may take effect, changing over time. Since they last decades to centuries, not all Characters in a Struggle are at War with each other all of the time.
Each Struggle consists of multiple Phases. As Characters within the Struggle Region act (e.g., declaring War, or Marrying), they may activate Catalysts that progress them between the different Phases.
Depending on the degree to which they're seen to be a part of the Struggle by other participants, Characters with Land in the area may be seen as Involved, Interlopers, or Uninvolved.
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.
Struggle is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Struggles are long-term conflicts involving large Regions with many Faiths and Cultures." It covers Struggle Phase, Struggle Catalyst, Involved, Interloper, Uninvolved and 6 more. The game also links it as Struggles and Struggle's. Patch 1.20.1.2, from game/common/game_concepts/03_fp2_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/03_fp2_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.