What does brick really mean?

[Astrak]

Okay, so I've heard the term "This will brick your phone" now I'm a Motorola technician and when we say a radio has been bricked...it really means that without the radio system admins 512bit code your radio will never be unlocked. Is this the same with cell phones? So if you accidently flash a GSM ROM/Radio does that mean your cell phone attempts to connect to a GSM network without proper authorization and the reply from the tower/system tells it to go into brick mode or is it something that has more to do with a hardware error that can be recovered from rather than replacing the phone.

I haven't done this I heed all warnings, I was just wondering how it works.