Irreversible means impossible to undo, reverse, or change back to the way it was before. When something is irreversible, it's permanent, like a one-way door that locks behind you.

Some changes in nature are irreversible. When you bake a cake, you can't unbake it back into eggs, flour, and sugar. When you burn a piece of paper, the chemical change is irreversible: you can't unburn it. Once a tree is cut down, you can't reattach it to its stump and make it grow again (though you could plant a new tree).

In medicine, doctors talk about irreversible damage when an injury is permanent. In environmental science, some changes to ecosystems can be irreversible if species go extinct or habitats are destroyed beyond recovery.

The opposite of irreversible is reversible: a reversible jacket can be turned inside out and worn either way, and a reversible chemical reaction can go forward or backward. But irreversible changes move in only one direction. Time itself seems irreversible: we can't go backward and relive yesterday.

An irreversible decision is one you can't simply undo, so it's wise to think carefully before making one.