The Maunder Minimum is the name given to the period roughly from 1645 to 1715 A.D., when sunspots became exceedingly rare, as noted by solar observers of the time. It is named after the later solar astronomer E.W. Maunder who discovered the dearth of sunspots during that period by studying records from those years. During one 30-year period within the Maunder Minimum, for example, astronomers observed only about 50 sunspots, as opposed to a more typical 40-50,000 spots.
The Maunder Mininum coincided with the middle -- and coldest part -- of the so-called Little Ice Age, during which Europe and North America, and perhaps much of the rest of the world, were subjected to bitterly cold winters. Recently published data suggests that the Sun expanded during the Maunder Minimum and its rotation slowed. A larger and slower Sun, it is speculated, might also mean a cooler Sun that provides less heat to Earth. (Just why the Sun expands and contracts is still a mystery.)
Whether there is a causal connection between low sunspot activity and cold winters is the subject of ongoing debate. Some scientists believe that solar activity drives climate change more than carbon dioxide does (see global warming).