Logic Board is most commonly used around here.

as others have already said Logic Board is the other commonly used term. I've heard Apple refer to it as the MLB or Main Logic Board, too.

the term motherboard originates from early days of computers where the main circuit board contained the CPU and other major "logic" or chips, and usually also expansion slots. You could plug "daughterboards" into those slots to expand the functionality of the computer - e.g. printer interface, sound card, graphics card etc. That was a distinguishing feature of a motherboard.

p.s. to get into even more background of the terminology, the "board" part of this comes from the general term Printed Circuit Board, or PCB. this is the physical, non-conductive board on which there are conductive traces connecting the various electronic components. Logic board is just another way of referring to the PCB containing the major "logic" or chips of the computer.

Motherboard when we used cards stuck in it, logic board, main board

Sometimes the motherboard is also called the mainboard. That's a pretty old term, but I've seen it pop back up in technical literature from time to time.

I have been an electrical engineering technician for 25 years and have called them "System Boards".

Logic Board

N.

The previous two are correct- Logic Board was mainly used as a term for MBs in earlier macs, however the term has stuck.

Dell calls them System Boards as well...

-- N

Mobo is an appropriate slang term. Smartly saves specious speech syllables.