This is the term used for the reduction in capacity seen when you don't completely discharge the batteries before recharging them. Almost all premature failure of rechargeable batteries can be linked to overcharging, heat, or reverse voltage due to over-discharging. This is not completely true, but clearly makes the point about how little information is available about battery chemistry. The smaller the crystal structure, the greater the energy transfer. On the other hand, overcharging batteries can cause permanent damage.
Heat is the enemy of batteries.
When you have fully charged a cell, but continue to pump energy into it, the energy must go somewhere. Where it goes is into the generation of heat. When you heat the cell by overcharging, you can change the chemical properties and lower the output voltage, thus reducing the cell's useful life. Even if you don't generate much heat, such as when you overcharge for long periods with a trickle charger, you still reduce the useful range of the batteries as noted above. As a final endorsement of rechargeable batteries, I want to remind you about the price we all pay when we throw away all those alkalines.