An advertising cost calculated by dividing the cost of one or a series of commercial by the size of the audience, expressed in rating points. For example, if the cost of a commercial is $50,000 and the rating for a program is 12, then the cost per point is $4,166.67 ($50,000 divided by 12).

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