"Contiguous" means that they aren't broken up by land or water. In the US, that would exclude Hawaii and Alaska. There are 48 contiguous/continental states.
No European nation built an "empire" by its proper definition, but Great Britain was the first to exert major power over the continent having the 13 colonies under their control for some time. Spain could also be seen as a major player as well as they owned much of the Midwest territory that was eventually bought by Thomas Jefferson that came to be known as the Louisiana purchase.
