What is a pure substance cannot be broken down into any other substances by chemical or physical means?

It is called an element.
Some pure substances, called elements cannot be broken down into other physical or chemical means. Individually or in combination, the elements form the object in the world around you! You might be surprised but there are only a few more then 100 elements. In fact you probably use only 30-40 of them daily.
However, scientists are creating more elements as day by day passes in their laboratory. So the number of pure elements might increase soon. Most of the new created elements, though, we won't use. It will be mainly for scientific research and/or other uses.