Regeneration is the process of regrowing or renewing something that was lost or damaged. When a starfish loses an arm to a predator, it can regenerate a new one over time. When a salamander's tail gets bitten off, it regenerates completely, sometimes growing back within weeks.

In biology, regeneration is one of nature's most remarkable abilities. Some creatures, like planarian flatworms, can regenerate so completely that if you cut one in half, each piece can grow into a whole new worm. Humans have limited regeneration: our skin regenerates after a cut, our liver can regenerate if part of it is removed, and we constantly regenerate blood cells. But we can't regenerate entire limbs the way some animals can.

The word also describes renewal in other contexts. A forest might regenerate after a fire, with new trees sprouting from seeds in the ash. A neighborhood undergoes regeneration when old buildings are restored and new life returns to the area. When doctors talk about regenerative medicine, they're working on treatments that help the body regenerate damaged tissues or organs, bringing hope to people with injuries that don't heal on their own.