Mainframes are powerful computers used mainly by large organizations for critical applications, typically bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.
A mainframe is a continually evolving general purpose computing platform incorporating in it architectural definition the essential functionality required by its target applications."
Some additional comments about this definition are in order. One of the most fundamental features of the mainframe world is the rapid and apparently endless evolution of the product line. From 16 general and 4 floating point registers of System/360, to the control register additions in the early 370s, to the access registers of the latter 370s, to the full complement of floating point registers of System/390 and the full 64 bit implementation offered by the z800/900 models; from 6 selector channels to 16 block multiplexing channels to 256 high speed optical channels; from 142 instructions to over 500 instructions; from real addressing to virtual addressing to virtual machines; from the simple 8 bit memory of the 360/30 through generations of development to the multiported, multilevel caching, multiprocessor supporting memory of the z900, the entire hardware domain of the mainframe world has been characterized by an unmatched, and indeed accelerating, evolution.
Mainframes used to be defined by their size, and they can still fill a room, cost millions, and support thousands of users. But now a mainframe can also run on a laptop and support two users. So today's mainframes are best defined by their operating systems: Unix and Linux, and IBM's z/OS, OS/390, MVS, VM, and VSE. Mainframes combine four important features: 1) Reliable single-thread performance, which is essential for reasonable operations against a database. 2) Maximum I/O connectivity, which means mainframes excel at providing for huge disk farms. 3) Maximum I/O bandwidth, so connections between drives and processors have few choke-points. 4) Reliability--mainframes often allow for "graceful degradation" and service while the system is running.