Game concept
Commoner
The game also links it as Commoners, Lowborn.
From the game's encyclopedia
A Commoner or Lowborn Character is not a member of a Noble Dynasty. They can, however, still be powerful Rulers of Theocracies and Republics.
If a Commoner becomes Landed, they immediately become Nobles and found their own Dynasties (except in Republics and Theocracies, which are largely run by Commoners).
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.
Commoner is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A Commoner or Lowborn Character is not a member of a Noble Dynasty." The game also links it as Commoners and Lowborn. 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.