Large and expensive computers historically associated with centralized rather than distributed computing. These computers are capable of supporting hundreds, or even thousands, of users simultaneously.

These are large computers - sometimes taking up a whole room! - that are usually found in big companies and colleges, and that are used by many people. Mainframes are expensive and often need special air-conditioned rooms. While many mainframes are still being used they are rapidly being replaced by smaller computers even by personal computers like yours.

the basic foundation on which the locomotive is constructed.