1. A multi-user computer designed to meet the computing needs of a large organisation. Originally, the term mainframe referred to the metal cabinet that housed the central processing unit (CPU) of early computers. The term came to be used gerneally to refer to the large central computers developed in the late 1950s and 1960s to meet the accounting and information-management needs of large organisations. The largest mainframes can handle thousands of dumb terminals and use terabytes of secondary storage. See minicomputer, personal computer, and workstarion. From QUECID

2. The largest and most powerful type of computer system that is widely used. A mainframe typically occupies many cabinets and fills an entire room. From Linux Guide @FirstLinux