It is called Flevoland, the 12th province.
The Dutch call their area of land reclaimed from the sea "polders." These are low-lying lands enclosed by dikes where water is drained to create arable land. Polders are a notable feature of the Dutch landscape and have helped expand their habitable territory.
ondergelopen grond
Growing crops
The dutch province of Flevoland was reclaimed from the 'Zuiderzee' (South Sea).
Land reclaimed from the sea is often referred to as reclamation land or land reclamation. It involves altering coastlines or waterways to create new land for various purposes such as urban development or agriculture.
in the Netherlands we call reclaimed land "polders"
polder
They reclaimed it from the North Sea, not land.
Lands reclaimed by the sea can be called either land fills or reclamation ground. In New Zealand, it is called land rehabilitation.
The Netherlands is known for reclaiming land from the sea through extensive land reclamation projects. Around half of the country's land is reclaimed from the sea, using systems of dikes, canals, and pumping stations to manage water levels and create new land.
The Dutch put windmills all over the land to clear the water on the surface. Know they have lots of dyke's to keep the water out.
I don't really know what 'land that was reclaimed' you mean, there are multiple areas where first was water and now is land. But if you mean "de Zuiderzee", there where first was sea and now is land (the biggest area that was reclaimed): The Dutch build a big dike (called "de Afsluitdijk"), making "de Zuiderzee" a lake and no longer a part of the sea. This lake, which was and still is called "het IJsselmeer" isn't completely made into land however, only some of the southern parts and some other smaller parts around the lake. These southern parts of the former "Zuiderzee" that now lie there are called "Flevoland" and the "Noordoostpolder". And if you simply mean how any reclaimed land would be called in the Netherlands, they usually call it a polder. I hope this answers the question.