Etymology: Sonder is a neologism, coined by John Koenig for his online Dictionary of Obscure Sorrows. As such it has no evolution to trace. A pleasing folk etymology might be the combination of the verb sound, to “ascertain the depth of water,” and wonder, an “astonishing or marvelous thing.” Many alternatives have been proposed by the dictionary’s devotees.

Meaning: Koenig’s definition begins: “The realization that each random passerby is living a life as vivid and complex as your own,” and expands into a brief but beautiful prose poem, like many entries in his dictionary. He likes to say that sonder is the revelation that you are merely an extra in someone else’s story.

Sonder shouldn’t be confused with empathy. Nor is it meant to convey a psychological theory of...