Someone with essentially no life, who spends 99% of their lives at home, indoors away from the rest of society. The only possible way of communicating with a homestucker is online, through text, or maybe you'll occasionally see them bulking up on supplies at your local grocery store.

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.