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. YMMV