Briefly, "the realization that each random passerby is living a life as vivid and complex as your own." Originally from the Dictionary of Obscure Sorrows, which has a lot more beautiful neologism definitions like this you might enjoy.

A catch all word to describe anything that is basic, uncool or untrendy. According to The New York Times, "cheugy (pronounced chew-gee) can be used, broadly, to describe someone who is out of date or trying too hard."

Things that are cheugy:

The New York Times
PT Cruisers
Goldendoodles

Australians

Addison Rae
White people
Billionaires

The person writing this definition

The realization that each random passerby is living a life as vivid and complex as your own — populated with their own ambitions, friends, routines, worries, and inherited craziness — an epic story that continues invisibility around you like an anthill sprawling deep underground, with elaborate passageways to thousands of other lives that you’ll never know existed, in which you might appear only once, as an extra sipping coffee in the background, as a blur of traffic passing on the highway, as a lighted window at dusk.

Sonder is the realization that everybody around you, even strangers, are living a life just as complex as yours. Sometimes your hurting so bad and you feel your the only one. But after all your hardships you start to realize that everybody is so unique and you aren't the only one. Everybody lives are so indescribably different and there is no way you will ever understand their lives. To sum it up you are realizing that your not the only one living with such chaos and complexity. All people lives are unique, strange, chaotic, and complex, just like yours. Your not alone.

the realization that each passerby is living a life as complex as your own.

origin: unknown

The realization that everyone around you has a life outside of yours, complete with their own thoughts, feelings, and emotions. You could potentially be just a passing figure on the street to them.

n. Pseudophilosophical guff memetically spread by the tumblr blog "The Dictionary of Obscure Sorrows". No academic grounding whatsoever, can be figured out through common sense.