Memory Loss in batteries refers to the loss in battery capacity due to incomplete discharging. In certain types of batteries, particularly Nickel Cadmium batteries, the capacity of the battery is reduced if the battery is not discharged completely. 
For example, if a battery is discharged up to 25% of its capacity and then charged up to 100% two or three times, the actual capacity of the battery will be reduced by 25%.
A film forms over the unused area of the anode which prevents the battery from completely discharging.
Memory Loss can also be caused by incomplete charging.
Memory Loss can be prevented by full charging and fully discharging the battery every two months.