means The main printed circuit board of a computer, into which the processor, memory, and other components connect and through which they communicate.

from A compound of "mother," used in the sense of a central or originating source from which others branch, plus "board," the flat panel of a circuit. The naming follows an old computing convention where smaller boards plugged into the big one were called "daughterboards" or "daughter cards" — making the motherboard the parent that births and binds the family. The term took hold as personal computers spread in the late 20th century.