The term is fairly new and as such the common definition is still changing, but Wikipedia agrees with regularfry

Patent troll is a pejorative term used for a person or company who enforces patents against one or more alleged infringers in a manner considered aggressive or opportunistic with no intention to manufacture or market the patented invention.*

http://en.wikipedia.org/wiki/Patent_troll

(this article is referenced: Alexander Poltorak. "On 'Patent Trolls' and Injunctive Relief"., ipfrontline.com, May 12, 2006)