Original meaning of "web portal" was coined in mid-90's based on analogy, that users would enter the virtual world (the web) stepping through the portal. Sounds totally cheesy nowadays, but that was the marketese of the 90's.
Basically that would be kind of site, which user would set as default in the browser and use it to navigate to other sites. At that time most portals would be aggregating all kind of content, and also would provide content directories (one of the last directories to be still alive is ODP (aka DMOZ)).
The problem with directories was that with the Web rapidly growing they couldn't keep up. They've been made obsolete by generic web search.
So really, what killed portals is Google. But on the other hand, google.com kind of fits wider definition of portal. It is a page that many users have set as their default page in the browser, and it is page that they use to navigate to other pages.
Other meanings - by late 1990's "portal" became most abused buzzword, so any startup building any kind of web site would call it "a portal" just to get piece of dot-com rush (which later was known as dot-com bubble).