The name for a period from CE 1790-1830 when sunspots were very rare. It is similar to the Maunder (CE 1645-1715) and Sp minima (CE 1460-1550) because it also coincided with a period of lower-than-average global temperatures.
Since no one has figured out what is causing this correlation, it is a key AGW-skeptic generator.
1. It was named for the English meteorologist John Dalton.