Game concept
Cultural Spread
The game also links it as Spread.
From the game's encyclopedia
Contact with other Cultures that have undiscovered Innovations will cause one of those Innovations to be discovered more quickly. Among eligible Cultures the one with the highest Cultural Acceptance will be selected to Spread Innovation progress.
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.
Cultural Spread is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Contact with other Cultures that have undiscovered Innovations will cause one of those Innovations to be discovered more quickly." The game also links it as Spread. 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.