Early 'computers' were massive structures, often occupying several
large rooms in a building, or even several buildings.
The word "unit" referring to a small room, or group of rooms, such as a motel "unit". The main section was the "Central Processing Unit" (CPU) where all the processing was done. Now days the CPU can fit in your hand and the whole computing facility has shrunk from building size to palm-top size - all in less than fifty years!