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In internet terminology, a troll is a person who posts inflammatory messages on the internet, such as on online discussion forums, to disrupt the discussion or to upset its participants. The word, or its variant, "trolling", is also used to describe such messages or the act of posting them.

Anyone who posts something that is wilfully stupid or intentionally provocative. It's called "trolling" in Internet-speak.
Sometimes, people who are merely attempting to be funny are accused of "trolling", when that is not their intent. Many trolls now find that the traditional trolling tactics are so overused and commonplace that they have to disguise their trolling to make it effective — quite often, the disguise involves accusing others of being trolls themselves.

Often the phrase "Don't feed the troll," gives trolls ammuntion; replying to their posts makes them stronger, and perpetuates the argument