WordNet (r) 3.0 (2006):

whistleblower
    n 1: an informant who exposes wrongdoing within an organization
         in the hope of stopping it; "the law gives little
         protection to whistleblowers who feel the public has a
         right to know what is going on"; "the whistleblower was
         fired for exposing the conditions in mental hospitals"
         [syn: whistle blower, whistle-blower, whistleblower]