Narrowcasting has traditionally been understood as the dissemination of information (usually by radio or television) to a narrow audience, not to the general public. Some forms of narrowcasting involve directional signals or use of encryption. In the context of out-of-home advertising, this term often refers to the display of content on a digital signage network.

Narrowcasting involves aiming media messages at specific segments of the public defined by values, preferences, or demographic attributes. Also called niche marketing or target marketing. Narrowcasting is based on the idea that mass audiences do not exist.

Narrowcasting is also sometimes applied to podcasting, since the audience for a podcast is often specific and sharply defined.

Narrowcasting is a form of broadcasting, if the latter term is understood as the "wide dissemination of content through mechanical or electronic media" as defined by Dr. Jonathan Sterne of McGill University.

Marketing experts are often interested in narrowcast media, since access to such content implies exposure to a specific and clearly defined prospective consumer audience. 

http://www.museum.tv/archives/etv/N/htmlN/narrowcasting/narrowcasting.htm 

http://www.teachradio.com/

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