As a Noun:
1. A mistake which is the result of failing to see or be aware of, or failing to realise the importance of, a fact or a problem.
2. Something which has not been included through a mistake or by being overlooked. English = omission.
As a Verb or Adjective:
1. To fail to see or be aware of, or fail to realise the importance of, a fact or a problem. English = overlook, disregard, neglect.
2. To forget to include something. English = omit.