Omission (noun)

1. The act of leaving out or excluding something.
2. The failure to do or include something that should have been done or included.
3. The state of being left out or excluded.

Origin:

From latin omissio (a letting go, a neglect), from omittere (to let go, to neglect).

Examples:

1. The omission of the important detail caused confusion and errors in the report.
2. The omission of his name from the list was a mistake that needed to be corrected.
3. The omission of the required steps made the instructions incomplete.
4. The omission of the vital information was a serious oversight.
5. The omission of the critical evidence weakened the case.