How does Nordic define flash memory endurance and retention?

[John Chinnick]

Is there an application note or specification that defines the term flash memory endurance and flash memory retention?

What failure modes can be expected when these specs are exceeded?

[Adrian Eggenberger]

The product specification gives the answers for your questions:

Endurance: 20000 Erase/Write cycles

Retention: 10 years at 40°C

[Stefan Birnir Sverrisson]

The type of failure is not specified when 20.000 erase cycles are exceeded. The flash may last longer but 20.000 cycles is guaranteed endurance. I suspect though that the failure will be that flash pages can not be properly erased or written anymore which will cause flash data to be incorrect.