Game concept
Fertility Equilibrium
The game also links it as Equilibrium.
From the game's encyclopedia
Fertility Equilibrium is a prediction of how fertile the County will become over a longer time. It is a value toward which County Fertility will arrive over time.
If the current County Fertility is below the Equilibrium, it will grow, and decline if it's above it.
The Fertility Equilibrium and the speed of change are mostly dependent on the ratio of Growth and Consumption modifiers for the County. If both Growth and Consumption are relatively small, any change will have a much greater effect on the Equilibrium.
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.
Fertility Equilibrium is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Fertility Equilibrium is a prediction of how fertile the County will become over a longer time." It is part of County Fertility. The game also links it as Equilibrium. Patch 1.20.1.2, from game/common/game_concepts/mpo_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/mpo_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.