Clearance means the amount of space between two objects, or official permission to proceed with something.

When you're driving under a bridge, the clearance is how much room exists between the top of your car and the bottom of the bridge. If a sign says “Clearance: 12 feet,” any vehicle taller than 12 feet won't fit. Construction workers need to know the clearance under power lines before operating tall equipment. Basketball players are lucky to have natural clearance when reaching for rebounds: the space between their outstretched hands and the rim is smaller than it is for shorter players.

The word also means official authorization. Airport security gives passengers clearance to board their flights after checking their tickets and bags. Before a rocket launches, mission control must give clearance, confirming that everything is safe and ready. A doctor might give you clearance to return to sports after an injury heals.

In stores, clearance refers to items being sold at reduced prices to clear them out and make room for new inventory. That clearance rack at the back of the store contains items the shop wants to sell quickly, often at steep discounts. The store is literally trying to clear space on its shelves.