“What aesthetic is this?” a commenter asks on a TikTok video of a person walking outside. “Walking outside” comes the response. The original commenter replies with a polite “Thank you!”—presumably before stepping outside to embark on a new aesthetic journey. Perhaps this exchange can be pinpointed as the moment when the word “aesthetic” stopped meaning anything.

“Aesthetic” does have a meaning, however. It refers to the study of beauty or a set of principles relating to an artist or movement. TikTok is not solely responsible for its more recent bastardization. That began with Tumblr, where online subcultures like “pastel goth” and “Lolita” dominated the early 2010s. The word brought together all the different aspects of a person’s real-life style, from th...