Ratings surveys in which local markets are simultaneously measured by a rating service. Nielsen Media Research surveys all 210 local television markets in November, February, May and July (Honolulu, Fairbanks and Juneau are excluded in July). These months are known as sweep months, and the data are used by local stations and cable systems to set local ad rates and to make program decisions. The term “sweep” dates back to the beginning of local television measurements in the 1950s and refers to how Nielsen Media Research mailed diaries to sample households starting with the East Coast and sweeping across the nation.
Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf