Brick

A brick is when a device no longer fully functions.

What is a brick

 * A console that can't turn on without additional hardware (for example, a 3DS might need an ntrboot cartridge to fix corrupted firmware partitions or a NAND header)
 * A console that can't boot to its main menu but can to homebrew utilities (like GodMode9) which can be used to fix the issue
 * A console that can turn on but is severely reduced in functionality due to a core component not working in stock firmware (such as the software keyboard on the 3DS)

What isn't a brick

 * A console that needs support files to fully turn on (for example, a 3DS might need  on the SD card that boot9strap can find)