citizen informant
noun
                                                                                                                            
                                                            citizen in·for·mant
                    
                                                                                                              
                                                                                                                              
            -in-ˈfȯr-mənt
                                                      
                                                          
                                
              
          
                                                      : an informant who is motivated to assist law enforcement officers by good citizenship as distinguished from one seeking some gain (as payment or concessions from police)                                      
                
called also citizen informer
Note: The information given by a citizen informant is presumed under the law to be reliable.
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  Merriam-Webster unabridged




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