POLDER (POLarization and Directionality of the Earth's Reflectances) is a passive optical imaging radiometer and polarimeter developed by the French space agency CNES.

A polder is a low-lying tract of land enclosed by embankments (barriers) known as dikes that forms an artificial hydrological entity, meaning it has no connection with outside water other than through manually operated devices. There are three types of polder:

• Land reclaimed from a body of water, such as a lake or the sea bed
• Flood plains separated from the sea or river by a dike
• Marshes separated from the surrounding water by a dike and subsequently drained