A moat is a deep, wide ditch dug around a castle or fortress, usually filled with water, designed to protect against attackers. In medieval times, moats made it extremely difficult for enemies to reach castle walls. Soldiers couldn't simply march up and break down the gate when a moat stood in their way. Some moats were so wide that even the longest ladder couldn't reach across them. Others had muddy bottoms or steep, slippery sides that made crossing nearly impossible.

Moats worked because they created distance and obstacles. Attackers would need boats or bridges to cross, giving defenders time to rain arrows or drop heavy objects from the walls above. Even if a castle's outer defenses fell, the moat gave those inside a powerful last line of protection.

Today, when people talk about a company having a moat, they mean it has something that protects it from competition, just like those medieval ditches protected castles. A company's moat might be a unique invention, a trusted brand name, or exclusive access to important resources. For example, a theme park with beloved characters that can't be copied has a strong moat around its business. The wider the moat, the harder it is for competitors to threaten its success.