The time between the delivery of a battery and its end of life (EoL).
A battery can still be used beyond this point, but no longer achieves the desired values and must be recharged significantly more often, for example.
Normally, the end of life is measured by falling below a predefined capacity.
The life of a battery depends on many factors, so specifics are usually approximate and only comparable when exactly the same conditions are used. Factors include battery type, battery state of charge, ambient temperature, number of cycles, depth of discharge, and to some extent time, regardless of battery usage.