The time segments that divide a radio or TV day for ad scheduling purposes. These segments generally reflect a television station’s programming patterns. The most common dayparts are: prime time, daytime, late night, early morning, total day, sign-on/sign-off, prime access and fringe. There is no universal agreement, however, about the exact times for all these dayparts, and for this reason, Nielsen Media Research reports the data by time of day.

Source:
http://www.nielsenmedia.com/glossary/
http://www.arbitron.com/downloads/terms_brochure.pdf