Impassable means impossible to travel through or across. When a road becomes impassable after a heavy snowstorm, cars can't get through because the snow is too deep. When a river runs so high and fast that boats can't cross it safely, that river is impassable.

Mountains can be impassable in winter. Dense jungle might be impassable without machetes to cut through the vegetation. A hallway piled high with boxes becomes impassable until someone clears a path.

Sometimes people use impassable to describe differences or barriers between people or ideas. Two friends might have an impassable disagreement so serious that they can't find common ground. But the word works best for physical obstacles: a flooded bridge, a locked door, a fallen tree blocking the trail.

The opposite is passable, which can mean either “good enough” (a passable grade on a test) or “able to be passed through” (a passable road). When hikers check trail conditions before a climb, they want to know whether the route is passable or impassable. That distinction can mean the difference between a successful adventure and a dangerous situation.