A mill pond is any body of water used as a reservoir

Reservoir

A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 for a water-powered mill

Watermill

A watermill is a structure that uses a water wheel or turbine to drive a mechanical process such as flour, lumber or textile production, or metal shaping .- History :...

. Mill ponds were often created through the construction of a mill dam across a waterway

Waterway

A waterway is any navigable body of water. Waterways can include rivers, lakes, seas, oceans, and canals. In order for a waterway to be navigable, it must meet several criteria:...

. In many places, the common proper name Mill Pond name has remained even though the mill has long since gone. It may be fed by a man-made stream, known by several terms including leat

Leat

A leat is the name, common in the south and west of England and in Wales, for an artificial watercourse or aqueduct dug into the ground, especially one supplying water to a watermill or its mill pond...

 and mill stream.

The term mill pond is often used colloquially to refer to a very flat body of water. Witnesses of the loss of RMS Titanic reported that the sea was "like a mill pond".