To me it is only a century if it is done as 1 ride or all done in the same "day". In this case "day" being midnight to midnight. No fair doing 50 miles in the afternoon and 50 miles the next morning and calling it a century. You can sort of call it a century if you ride 50 miles in the morning and eat lunch and then do 50 miles in the afternoon. But that is pushing the envelope.

I prefer calling centuries, centuries only if they are done as a single ride. You can stop and take a break or even stop and eat a meal. But you are still on the same ride.