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.
I felt deep sonder thinking about the all the people who looked up this word along with me.
by rednos January 10, 2013
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.
Sam and I were sondering down the street one day.
by shrek's Wife November 20, 2012