1. (noun)mainframe, mainframe computer a large digital computer serving 100-400 users and occupying a special air-conditioned room
2. (noun)central processing unit, CPU, C.P.U., central processor, processor, mainframe (computer science) the part of a computer (a microprocessor chip) that does most of the data processing
"the CPU and the memory form the central part of a computer to which the peripherals are attached"