Plagiarism is simply the use of a source without attribution; passing others' work and ideas off as one's own. But on Wikipedia, we should not be passing off ANYTHING as our own (No original research). Thus it can be seen the Wikipedia goal is different than that of an academic paper or suchlike. In academia, the crime is one of claiming credit to oneself that deserves to go to another. On Wikipedia, we're not writing for individual credit.
Thus, it is unsurprising that Wikipedia has no explicit plagiarism policies; it is subsumed under WP:NOR and WP:CITE. Although, as Fred Bauder says, we probably should have a page on it in policy anyway, so as not to confuse people coming from outside.
In general, if a Wikipedia article (or a section of it) is a paraphrase of another work, that may mean that insufficient sources were used in its writing. That can be corrected.
-Matt (User:Morven)