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Though portals are particularly poorly defined, two definitions are useful. First, Jakob Nielsen defined a portal as "the big three": news, search, and directory. Builder.com classified different types of portals, ultimately claiming that "the most popular features of a portal are Web searching, news, reference tools, calendaring, and communication capability."
The Zope Portal Toolkit (PTK) project uses a definition that leverages what's good about Zope and the patterns Digital Creations sees in its consulting practices. Thus the PTK has a portal vision.