Any computer you can't lift. Generally requires a raised floor and dedicated air conditioning. Some good examples are big iron are:
- Sun 690MP
- Sun Ultra Enterprise 500 and up
- Most of the VAX machines; especially the VAX 11/780
- Silicon Graphics Origin 2000, Onyx, etc.
The true definition of big iron varies, of course. I call anything a well muscled person can't lift big iron. Some people require that a machine require three-phase power
before receiving that title