Happiness
Happiness (colony) in the Stellaris encyclopedia: Happiness is a measure of the overall satisfaction of Pops, influenced by factors such as Living Standards, Edicts, and various Planet Modifiers.
Happiness in the game's words
The encyclopedia entry as the game shows it in its tooltips; underlined words open their own entry or page here.
Happiness is a measure of the overall satisfaction of 
Pops, influenced by factors such as Living Standards, Edicts, and various Planet Modifiers.
High Happiness increases 
Stability which in turn increases 
Job output, while low Happiness decreases Stability.
Combined with 
Political Power, Happiness determines the Pop Approval Rating.
Pops that lack Happiness count as having a constant 50% Happiness.
Entries that mention Happiness
More in Colony
Source: common/game_concepts (concept_happiness) and localisation/english, Stellaris Cygnus v4.5.2 (9776).