a nonclustered index that contains ALL the columns referenced in any clause of a query
                                                            
                                                     
                        
                                                
                        
                        
                            a non-clustered index that contains all the information requested by the user in the index's leaf level (including the key and the pointer)
                                                            
                                                     
                        
                                                
                        
                        
                            a nonclustered index that is built upon all of the columns required to satisfy an SQL query, both in the selection criteria and the WHERE predicate