Game concept
Terrain
The game also links it as Terrains.
From the game's encyclopedia
Each Barony has a specific dominant Terrain which can affect things like Supply, movement speed of Armies, the efficacy of the various types of Men-at-Arms Regiments in Battle and the Buildings that can be constructed there.
Many Commander Traits also deal with Terrain.
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.
Terrain is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Each Barony has a specific dominant Terrain which can affect things like Supply, movement speed of Armies, the efficacy of the various types of Men-at-Arms Regiments in Battle and the Buildings that can be constructed there." The game also links it as Terrains. 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.