The increasing number of audience subdivisions which, together, constitute total TV usage. Television audiences are said to be fragmented, for example, across a broad spectrum of video sources: nine broadcast networks, more than 50 cable networks, hundreds of syndicated programs, new DBS services, VCR and video game usage, Internet usage, etc.

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