Serotonin
Etymology
From "sero-" (from serum, the fluid part of blood) and "tonin" (from "tone"), because it was first isolated in blood serum and found to affect the tone, or tightening, of blood vessels. Only later was its role in the brain recognized.
noun · /sair-uh-TOH-nin/
A body chemical that helps regulate mood, sleep, appetite, and digestion.
What it names
A handle for the steadying, background chemistry of contentment and rhythm — not a quick reward signal.
Example
Many antidepressants work by keeping more serotonin available between brain cells.
Related idea
If dopamine drives the pursuit of a reward, serotonin is more associated with mood stability and satisfaction with what you already have. Notably, most of the body’s serotonin lives in the gut, not the brain, governing digestion.