The Collaborative International Dictionary of English v.0.48:
Polder \Pol"der\, n. [D.] A tract of low land reclaimed from the sea by of high embankments. [Holland & Belgium] [1913 Webster]WordNet (r) 3.0 (2006):
polder n 1: low-lying land that has been reclaimed and is protected by dikes (especially in the Netherlands)