Something that can be replaced through a cycle quickly
Many energies that are alternatives to fossil fuels are renewable. These are energies that can be quickly replaced when we use them. These include: Biofuel, Hydroelectric Power, Wind Energy, Geothermal Energy, and Solar Energy.
Other Renewable Resources
Many other things are renewable as well since they are easily replaced through different cycles.
They are replaced through new plants through a life cycle.
They are replaced with offspring through a life cycle. Humans are renewable too!
The air we breathe is always being replaced through the Carbon Dioxide - Oxygen cycle.