(1) In media ratings, the unduplicated number of individuals or households exposed to an advertising medium at least once during the average week for a reported time period.  It is interchangeable with Cumulative Audience and Net Reach. (2) In internet usage, Reach is the percentage of U.S. Internet users that have accessed the Web content of a specific site or property.

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