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.