Costly means expensive or requiring a high price, whether in money, time, effort, or other resources. When something is costly, you have to give up a lot to get it or keep it.

A costly mistake is one that creates serious problems. If a soccer player makes a costly error in the final seconds of a championship game, their team might lose because of it. If a student forgets to save their project file before the computer crashes, that's a costly mistake because they lose hours of work.

In stores, costly items have high price tags. A costly repair on a car might cost thousands of dollars. A costly vacation requires saving money for months or even years.

The word often emphasizes what you're sacrificing or losing. A lie might seem harmless at first, but it can prove costly if it destroys a friendship. Staying up too late playing video games on a school night is costly because you pay for it the next day with exhaustion and trouble concentrating.

When someone says a decision was too costly, they mean the price (whether in money, time, relationships, or opportunities) was higher than the benefit.