(1) Grouping several commercials together during one break.
(2) In statistical terms, a group of a statistical population. Alternately, a cluster can be the classification of demographically similar geographic areas into one or more homogeneous groups. Each group represents distinctive lifestyles patterns and offers a basis for segmenting the market.

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