The sum of the percent of households or persons tuning or viewing during the average minute of each telecast of the program, including repeat telecasts during the report interval. Duplicated tuning and viewing to the same program (or its repeat telecast) by the same household during the report period is counted each time.

Source:

http://www.nielsenmedia.com/glossary/

http://www.arbitron.com/downloads/terms_brochure.pdf