Inconceivable means impossible to imagine or believe. When something is inconceivable, your mind simply can't wrap around it. It's beyond what you can picture or accept as real.

If someone told you that a classmate memorized every word in the dictionary overnight, you might say “That's inconceivable!” You're saying it seems so far beyond possible that you can't even form a mental picture of it happening.

Scientists sometimes use this word when discussing concepts that challenge our everyday understanding. Before the invention of airplanes, human flight seemed inconceivable to most people. They couldn't imagine how a machine heavier than air could stay aloft. Today, quantum physics describes particles being in two places at once, which many people find inconceivable even though experiments show it can happen.

When something is merely difficult to believe, you might call it unlikely or improbable. When it's inconceivable, it feels impossible to even think about seriously. It's the difference between “I doubt that happened” and “There's no way that could have happened.”