A widely used rating term, expressed as a percentage, to reflect viewing to the average minute of a program or time period. It is an average of the audience at minute 1, 2, 3, etc. As such, it serves as an estimate of the average commercial audience (households or persons).

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