Sonder
Etymology
An honest caveat: this is not an old word. It was invented in 2012 by John Koenig for his Dictionary of Obscure Sorrows, which coins terms for feelings that lack them. He assembled it to sound plausibly rooted (echoing French sonder, "to probe," and German sonder, "special"), but its origin is deliberate invention, not organic history.
noun · /SON-dur/
The realization that every stranger you pass is living a life as vivid and complex as your own.
What it names
That sudden, dizzying shift on a crowded street when the extras in your life reveal themselves as protagonists of theirs.
Example
Stuck in traffic, she felt a wave of sonder — a thousand people around her, each mid-story.
Related idea
It belongs with other modern coinages for un-named feelings; unlike them, it has quietly entered real use.