Game concept
Interloper
The game also links it as Interlopers.
From the game's encyclopedia
When a Character is tangled up in a specific Struggle but not yet considered fully Involved, they're an Interloper.
Interlopers have their Capitals within the Struggle Region, but do not have both an Involved Faith and an Involved Culture.
Fully Involved Characters think of them as nosy, troublesome outlanders who don't understand where they're not wanted. Interloper Rulers thus generally take some penalties in Counties they hold inside a Struggle Region.
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.
Interloper is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "When a Character is tangled up in a specific Struggle but not yet considered fully Involved, they're an Interloper." It is part of Struggle. The game also links it as Interlopers. 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.