Interesting snippet from the world wide wiggle:
System Boards
As with many aspects of computers and computing, large organizations are apt to apply their own nomenclature to things. IBM developed its own name for the board that held the principal circuitry of its entire line of personal computers from the original IBM PC through its successors XT and AT. Consequently what most people call a motherboard, IBM usually terms a system board. IBM’s choice of name is apt because the board (or more correctly, the circuitry on the board) defines the entire computer system. And, yes, one reason for using the term was to create a gender-neutral term. IBM didn't want to take sides in the war between the sexes. Other than word choice, however, there’s no difference between a system board and a motherboard in a PC.
This interchangeability of terms does not extend to devices other than PCs. Although a distortion analyzer, oscilloscope, or even television set may be built with a motherboard and daughterboards, you would never call the central printed circuit board in one of these devices a system board.
Planar Boards
Another gender-neutral term promoted by IBM for the motherboard, which first came into common parlance with the introduction of the Personal System/2 line of machines, was planar board. In conversation, IBM engineers often shorten the term to the simple adjective, planar. The term probably refers to the motherboard defining the principal plane of the computer system.
As with most gender-neutral neologisms, "planar board" is less descriptive than the terms it is meant to replace. At face value, the term could not be more vague or all-embracing. All printed circuit boards are planar—that is, flat—except, perhaps, for a few special-purpose flexible assemblies like those folded into cameras. Even the term "system board" is more precise in that it at least describes the function of the circuit assembly.
Baseboards
Just as IBM gives its own names to motherboards, Intel does likewise. The company’s preferred term (as seen in its technical manuals) is baseboard. On Intel technical literature, the term is given as one word and used interchangeably with motherboard. For example, the manual dated May, 1996, for the VS440FX lists the product as a "motherboard" while the manual for the Performance/AU data December, 1995, terms the product a "baseboard."
Main Board
Another gender-neutral term that some manufacturers use for motherboard is main board. The term is appropriate—the main board is the largest circuit board inside—and the foundation for—the computer system and, hence, it is the main board in a PC’s case. The main issue is, of course, whether we need yet another politically correct term for motherboard.
Logic Boards
The PC industry has no monopoly on vague, gender-neutral terms. In the realm of the Apple Macintosh, the main circuit board inside a computer is often called a logic board. Of course, every circuit board inside a computer is based on digital logic, and the term could describe any digital circuit board. When Apple people talk amongst themselves, however, they know what they mean when they say "logic board," and now you are privy to their secret.
Backplanes
Another name sometimes used to describe the motherboard in PCs is backplane. The term is a carry-over from bus-oriented computers. In early bus-oriented design, all the expansion connectors in the machine were linked by a single circuit board. The expansion boards slid through the front panel of the computer and plugged into the expansion connectors in the motherboard at the rear. Because the board was necessarily planar and at the rear of the computer, the term "backplane" was perfectly descriptive. With later designs, the backplane found itself lining the bottom of the computer case.
Backplanes are described as active if, as in the PC design, they hold active logic circuitry. A passive backplane is nothing more than expansion connectors linked by wires or printed circuitry. The system boards of most personal computers could be described as active backplanes, though most engineers reserve the term "backplane" for bus-oriented computers in which the microprocessor plugs into the backplane rather than residing on it. The active circuitry on an active backplane under such a limited definition would comprise bus control logic that facilitates the communication between boards.
