An interval of time on a station, cable system or network. Sometimes, but not necessarily, an entire Daypart (prime time, daytime, etc.). In audience research, ratings for time periods are often calculated, as opposed to program averages, for purpose of evaluating station or network performance.

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