A stimulus that elicits pain, fear, or avoidance.
                                                            
                                                     
                        
                                                
                        
                        
                            In instrumental conditioning, a stimulus such as an electric shock, which the animal does everything to avoid and nothing to attain.
                                                            
                                                     
                        
                                                
                        
                        
                            any negative stimulus to which an organism will learn to make a response that avoids it