A hard reset is any kind of abrupt power-down in which a computer isn’t allowed or able to go through the normal shutdown process of terminating applications and processes, closing files, completing pending filesystem transactions and so on.
There are a number of possible variations of the hard reset, such as:
- the external power failure (which can be largely mitigated with a UPS)
- the "forced reset", where the operating system becomes unsalvageably frozen and the user is obliged to force an immediate power-down with the power button or physical reset switch
- the "plug yank", in which the system becomes so throughly wedged that even the power button and physical reset switch stop working; in this case literally yanking the power lead may have some therapeutic value
I maintain a journal of my experiences with such failures in the Involuntary Reboot Log.