Unimaginable means impossible or nearly impossible to picture in your mind, usually because something is so extreme, so different from normal experience, or so overwhelming that your brain struggles to grasp it.

When astronomers say the universe contains an unimaginable number of stars, they mean the quantity is so enormous that you can't really form a mental picture of it, even though you understand the concept. The suffering during a major war might be described as unimaginable because it's so far beyond everyday experience that people who haven't lived through it can't fully conceive what it was like.

Sometimes people use unimaginable more loosely to mean simply “very surprising” or “hard to believe.” You might say it's unimaginable that your cautious friend tried the giant water slide, when really you just mean you're shocked because it's so unlike them.

When something truly is unimaginable, your imagination hits a wall. You know intellectually that a billion is a thousand million, but can you actually picture a billion of anything? That's what makes the number feel unimaginable.