Definition of mainframes:
(n) :
(computer hardware) A large, powerful computer able to manage very many simultaneous tasks and communicate with very many connected terminals; used by large, complex organizations (such as banks and supermarkets) where continuously sustained operation is vital.
(n) :
(computer hardware) A computer chassis that hosts and interconnects modules that perform various tasks.
(n) :
The main frame of a vehicle etc., more commonly main frame.