Electronic – How is the smallest erasable unit of flash memory called

To amplify stevenvh's answer, any type of logic, when given an input signal, will take some time to produce an output signal; memory is often very slow compared with other logic. Often, there will be a guarantee that the output signal will become valid within a certain amount of time, but that's it. In particular, it's possible that the signal might change several times within that interval, and there will be no indication, prior to the end of that interval, that the signal has achieved its final "correct" value.

When a typical microcontroller or microprocessor reads a byte (or word, or whatever unit) of memory, it generates an address and, some time later, looks at the value output by the memory and acts upon it. Between the time the controller generates the address and the time it looks at the value from memory, it doesn't care when or whether the output signals from the memory change. On the other hand, if the signal from memory hasn't stabilized to its final value by the time the controller looks at it, the controller will misread the memory as having held whatever value was being output at the moment it looked. Normally the controller would look at the value from memory as soon as it was ready to do something with it, but if the memory's value wouldn't be ready then, that might not work. Consequently, many controllers have an option to wait a bit longer after they're ready to process data from memory, to ensure that the output from memory is actually valid. Note that adding such delay will slow things down (the controller would have been happy to act on the data from memory sooner), but will not affect correctness of operation (unless things are slowed down so much that other timing obligations cannot be met).

I can't speak about FRAM (ferroelectric memory), but any technology that uses floating gates to store charge — any form of EPROM, including EEPROM and Flash — relies on electrons "tunneling" through a very thin insulating silicon oxide barrier to change the amount of charge on the gate.

The problem is that the oxide barrier is not perfect — since it is "grown" on top of the silicon die, it contains a certain number of defects in the form of crystal grain boundaries. These boundaries tend to "trap" the tunneling electrons more or less permanently, and the field from these trapped electrons interferes with the tunneling current. Eventually, enough charge is trapped to make the cell unwritable.

The trapping mechanism is very slow, but it is enough to give the devices a finite number of write cycles. Obviously, the number quoted by the manufacturer is a statistical average (padded with a safety margin) measured over many devices.