What might be the origin of the word "mainframe" for describing a type of computer?
The word is formed (obviously) from the adjective main applied to frame, which tends to coalesce into mainframe in any context where a particular main frame becomes a standard component and needs a noun for convenient reference. For example, the term was used for the main frames of locomotives and early automobiles. A particularly relevant such use is in telephone infrastructure, where there was a large frame used for an exchange, holding wiring and relays for a group of phone lines with a common prefix. For instance, in this 1918 article, the diagram has an arrow labeled "to new exchange mainframe".
Early computers used a large frame (much larger than "racks" used now) to hold wiring and components for part of a computer (like that shown here). By the late 1950s, a typical installation might have a single large frame holding the central processing unit, to which bundles of wires from peripherals ran. To those familiar with the similar usage in telephone technology, mainframe would have been a natural usage for this CPU frame. For example, in this 1961 article, a Western Electric (of AT&T) staff member writes:
Likewise, the output of a computer mainframe is digital...
The term became more widely adopted, in particular for the large central computer used by a company as distinguished from smaller computers which began to appear in the 1960s. It is not terribly surprising that the earliest source quoted by the Oxford English Dictionary for mainframe is a 1964 glossary from Honeywell, then a producer (among other things) of smaller computers and soon minicomputers.
Actually, IBM did use the term right from the beginning. Gene Amdahl (one of the lead designers of the System/360 mainframe) used the term in 1964 in an IBM journal describing the S/360 architecture.
- see G.M. Amdahl, G.A. Blaauw, and F.P. Brooks, Jr., "Architecture of the IBM System/360," IBM JOURNAL OF RESEARCH AND DEVELOPMENT, 8, no. 2 (1964)
"central processor of a computer system," 1964, from main (adj.) + frame.
In the days when large installations were becoming more common, a computer system would require a specially built room (as far as I know, they still do). Tape drives, disk drives, printers, card readers and the like would be distributed about the space. The "mainframe" refers in such cases to the processing unit and physical memory - which in the case of an IBM 360-158 was about the size of a commercial refrigerator.
Frame in the mechanical sense of board - although in the early machines it would be a framework of wired connections rather a than a printed circuit board.
The main frame was the central piece containing the processing unit, with other auxiliary frames containing memory, I/O, peripherals etc.
The first mainframe in modern terms was probably the IBM360 in the mid 1960's but IBM never used the term - in fact IBM pretty much avoided the term computer at the time.