Decommission means to withdraw from service. In an energy context it normally refers to nuclear decommissioning -- removing a nuclear energy plant from service. Because of the possible presence of radioactive or fissile material in a nuclear facility, special precautions are required.
Methods
Dismantling
Parts of the reactor are removed or decontaminated soon after the nuclear plant closes,allowing the land to be used for other purposes.
Safe Storage
The nuclear plant is monitored and radiation is allowed to decay. Then afterwards it is dismantled.
Entombment
Sealing off radioactive components with concrete and steel. This allows radiation to decay until the land can be used for other purposes.