If you make an omission, you leave something out. If you are a restaurant reviewer and you give a glowing review to a new restaurant but fail to mention that you own half of it, that's a significant omission.
The noun omission comes from the verb, omit, which means to leave out. Omissions can be purposeful or a result of neglect. Was the omission of Aunt Suzy from your wedding invitation something you meant to do, or an accident? If you're cleaning the house but don't take out the garbage, you'll regret your omission when the garbage is stinking up the kitchen.