What is an act of omission?

Sin of omission is one that is wrong because you failed to do something that you should have done. A better example is that when you were asked a question about something that happened, you didn't lie about what happened, you just didn't say that something did happen. For example, you are describing an argument that happened between Bob and Sue. You tell all about the fact that Bob called Sue some aweful names and that Sue responded by slapping Bob across the face. Then the argument was done. However, you didn't tell anyone that, after Sue slapped Bob, she told him that if he ever talked to her again, she was going to kill him. That would be a sin of omission. Better yet, you are working as a check-out clerk at a store. YOu see the clerk at the next aisle give the customer $10 too little change, and then pocket the money. You go home and don't say anything. You have done your job exactly as it should be done. However, you know that you should have reported the other clerk. That is a sin of omission.