A Transformer Core is Laminated to?

A transformer core is laminated to allow reduction of eddy current losses. 

The reduction in area of each part is achieved through lamination, this increases the resistance which gets quite high and therefore reduces the loss of eddy current to the minimum. It becomes more suitable for the magnetic flux to travel through the iron than coil or air, as small gaps are created between the plates due to the laminations and therefore the leakage flux or stray flux is highly reduced which minimizes the core losses and improves efficiency of the transformer.