The battery memory effect is a reduction in the longevity of a rechargeable battery's charge, due to incomplete discharge in previous uses.
Some types of batteries, such as nickel-cadmium and nickel-metal hydride, can develop a memory effect when only partially discharged before recharging. This “memory” reduces the capacity of subsequent charges and thus future battery life in a given device. The effect can also be caused by poorly-designed chargers.
The memory effect can be mitigated by diligent battery use: fully discharging and recharging. One can recondition batteries with a memory effect by deep discharge and charge cycles. Some chargers have specific battery reconditioning modes as well.
Newer types of batteries, such as those based on lithium ion and lithium polymer, offer better capacity and an improved ability to hold a charge while not in use, while also not demonstrating a memory effect when partially discharged.
In general, most people with a decent charger don't have to worry about memory effect any longer. Only consistent partial charges tend to produce the effect. In fact, draining the capacity completely is worse for the cells’ overall longevity and only advisable by those seeking maximum voltage for purposes such as remote control operations.