How do you define a renewable energy source? Why are coal, oil, and natural gas not renewable energy sources?

A renewable energy source is one that can not depelted in a reasonable timeframe.

Sun energy will last until the Sun dies, which is expected to haen in about 5 billion years. Because it will last so long it is considered practiacally eternal and undepleteable.

Fossil fuels on the other hand are material deposits in the crust of Earth that might be depleted within centuries.