Brick

The term "bricked" refers to nonfunctional device states. Though nominally meaning that the device is permanently damaged, in practice it includes conditions which range from trivially recoverable (a failed update) to completely unrecoverable (in certain cases involving damaged baseband memory). A phone may be referred to as "bricked" if it will not boot, will not respond to input, will not make calls, etc. Early unlock solutions frequently resulted in partially-bricked phones after firmware updates were applied. Generally speaking, the iPhone is quite difficult to brick permanently via software, and in almost all cases the damage can be reversed.

Irreversible Software Bricking

The only way to irreversibly brick your iDevice in software is to flash an invalid baseband bootloader, provided it has a baseband. All other bad flash scenarios are recoverable some way or another. It is impossible to brick your device simply by jailbreaking it, since DFU Mode will still be available to recover from a bad flash.

If you purposefully erase / zero out your NOR, then you will even have trouble doing a DFU restore because important information from the SysCfg section will not be available.