A priceless thing is something that has no value or price because it's so special or unique. It can't be measured with money.
When people say something is priceless, they usually mean it's irreplaceable or impossible to put a price on. For example, if you see an old painting in a museum and someone says it's priceless, they probably mean that it's a one-of-a-kind masterpiece that can never be made again, no matter how much money you spend. This word is often used when talking about something rare or of great sentimental value.
So precious as not to be sold at any price; invaluable.
