From Meta, a Wikimedia project coordination wiki

Image of Wikimedia Portals Portals are pages intended to serve as "Main Pages" for specific topics or areas. Portals may be associated with one or more WikiProjects; unlike WikiProjects, however, they are meant for both readers and editors of Wikipedia, and should promote content and encourage contribution. Portals are situated in the a special "Portal:" namespace.

The term portal may also refer to the point of entry to specific Wikimedia projects, like http://www.wikinews.org or http://www.wikipedia.org. See project portals for the list of current portals.