The condition of rechargeable nickel cadmium and nickel metal hydride batteries in which it continues to hold less of a charge over time. It is said to "remember" how full it was when last charged because it will not charge past that point the next time. This is why you should completely drain nickel-based batteries every month or so.
The memory effect is caused by a combination of chemical reactions; however, the cadmium in a nickel cadmium battery is the bigger problem and why nickel metal hydride batteries fare somewhat better. In a fresh battery, the anode's cadmium crystals are approximately one micron across. If the battery sits in the charger too long or is not fully discharged, over time, the crystals grow to as much as 100 microns. This conceals more of the active material to the electrolyte and reduces battery life. See
batteries.