Nebulous is a formal word used to describe something that is difficult to see, understand, or describe—in other words, something indistinct or vague.

// A lot of philosophical concepts can seem nebulous at first, but a good instructor can cut through the jargon and help students see how they apply to day-to-day life.

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“[Rob] Harvilla began to notice the blurred lines of late-Nineties genres as he produced his podcast 60 Songs That Explain the ‘90s and while writing its corresponding book. ‘The late Nineties were a weird, transitional wasteland,’ he says. All of these genres that had such stark lines in the Nineties have now become a more nebulous concept, blending into one supergenre of just ‘Nineties music.’” — Brittany Spanos, Rolling Stone, 21 July 2025

Nebulous may sound otherworldly—after all, it’s related to nebula, which refers to an interstellar cloud of gas or dust—but its mysteriousness is rooted in more earthly unknowns. Both words ultimately come from Latin nebula, meaning “mist, cloud,” and as far back as the 14th century nebulous could mean simply “cloudy” or “foggy.” Nebulous has since the late 17th century been the adjective correlating to nebula (as in “nebulous gas”), but the word is more familiar in its figurative use, where it describes things that are indistinct or vague, as when Teju Cole wrote of an avant-garde photographer who viewed photography as existing “neither in the camera nor in the printed photograph, but in a more nebulous zone.”