What is the memory effect for rechargeable batteries?

The memory effect is an occasional defect, which occurs in Ni-Cd and Ni-MH rechargeable batteries, where the maximum battery voltage decreases even though the original power of the battery remains the same.

In particular, this could occur when batteries are charged whilst still holding some power. This way the battery memorises the fact it hasn’t been used to its maximum capacity, after which the battery’s capacity drops when used the next time despite it being fully charged.
The memory effect doesn’t affect the performance of lighting equipment, electric toys, etc.
However, when batteries with memory effect are used in digital cameras, it could induce a decline in operating time depending on cut-off voltage whereby the camera determines it has no power left. 

If the memory effect occurs, discharge the battery by inserting it in a flash light (or something similar) until the battery is completely drained of its power. Then fully recharge the battery to its maximum capacity to bring it back to its original performance. 

eneloop is especially designed not to suffer from this memory effect.