Slang for large, expensive, ultra-fast computers or systems. Associated with number-crunching supercomputers such as Crays, but also include more conventional big commercial IBMish mainframes and high-end servers for large companies or organizations with deep pockets or computation-intensive needs. Term of approval; compare heavy metal, oppose dinosaur.
While speed and I/O throughput is a major concern with Big Iron, these days the main concern with machines of this type seem to revolve around ultra-high availability and reliability. For high-performance number-crunching, large farms of cheap computers clustered together seem to be the way to go these days, rather than a big freon-cooled box that sucks more electricity than an aluminum smelter...
Also see Big Iron Aint Cheap, Big Iron Database, www.catb.org 
See original on c2.com