In an argument (usually a political debate), a concern troll is someone who is on one side of the discussion, but pretends to be a supporter of the other side with "concerns". The idea behind this is that your opponents will take your arguments more seriously if they think you're an ally. Concern trolls who use fake identities are sometimes known as sockpuppets.

A person who posts on a blog thread, in the guise of "concern," to disrupt dialogue or undermine morale by pointing out that posters and/or the site may be getting themselves in trouble, usually with an authority or power. They point out problems that don't really exist. The intent is to derail, stifle, control, the dialogue. It is viewed as insincere and condescending.

A person who lurks, then posts, on a site or blog, expressing concern for policies, comments, attitudes of others on the site. It is viewed as insincere, manipulative, condescending.

In a situation where there exists mutually exclusive positions A and B, a concern troll is someone who supports A but professes to support B around genuine supporters of B. However, they express their "concerns" about aspects of position B in order to sow doubt and uncertainty amongst genuine supporters of B.

Whilst this does genuinely happen, the term is used by some paranoid people to effectively mean "anyone who does not agree entirely with the standard dogma of position B, thus must actually covertly support position A", when said person does actually support B, just only 99%.

Thus: disagreeing *at all* with the standard dogma of B will get you branded by such people as The Enemy, and you will get generally treated with hostility and suspicion and, at worst, forcibly ejected from the B-supporter group. Result: everyone stops thinking about the issues, and discussion descends into not arriving at logical and interesting places but who can most furiously support B.

Someone who raises a false question as a means of injecting a totally unsupported allegation into the public discourse. When challenged on the barely-veiled insinuation, they claim that they were "only asking a question."

A term of abuse. Mostly used in forums where there is a fairly well-defined orthodoxy, usually political (it could be conservatism, feminism or nearly anything else) that all members are assumed to agree with by default, "concern troll" can refer to nearly anyone who expresses disagreement or skepticism about some aspect of that orthodoxy, while agreeing with other parts of it.

Supposedly, the idea is that the "concern troll" is actually an adherent of some other, opposing orthodoxy, disingenuously pretending to be sympathetic to the goals of the forum in order to disrupt it or sow dissent. Perhaps this actually happens (this longtime forum user is skeptical, having NEVER seen a clear example), but usually, the accusation comes from someone who can't imagine honest disagreement with his or her favored ideology, and thus says more about the accuser than the accused. Basically it's a bludgeon used by people with very black-and-white views on some topic, against anyone more nuanced than themselves.

Accusing someone of being a concern troll is generally a bad idea. Even if you're right about the "troll"'s motives, which you probably aren't, that doesn't make the "troll"'s arguments wrong; in other words, calling someone a concern troll is a basic ad hominem fallacy. The term is used to shut down, rather than to advance, discussion.

A sleazy way to discredit anyone who disagrees with your point of view, especially if that point of view can cause physical harm to others.