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An informant (sometimes informer or intelligencer) is someone existing inside a closed system who provides information of that system to a figure or organization who exist outside of that system. Most notably these organizations include law enforcement agencies, but also informants are utilized by others such as social scientists.
Phrased less formally, an informer or informant could be a member of an underground organization, a criminal gang or any other group outlawed, persecuted or harassed by the civil or military authorities, who gives the authorities information about the group as a whole and/about other members.
Public attitudes to informers and informants change enormously with different circumstances. When the group concerned is involved in principled opposition to a tyrannical regime or a foreign occupation, an informer within its ranks will likely to be considered (at least by those sympathizing with its aims) as a despicable traitor. On the other hand, in cases of criminal gangs considered a danger to society, use of informers might be considered socially useful. In cases where the role of an organization is debated - for example, the many groups regarding themselves as "freedom fighters" but defined by the authorities as "terrorists" - appreciation of informers within their ranks may vary accordingly.