The words are supposed to be there but have been left due to any reason. Is there a succinct term to describe such words?
There's some disagreement going on here that can be attributed to differing interpretations of the question.
If the text was once there but is now missing or illegible, it is a lacuna.
If a word or phrase was left out unintentionally, it is an omission or elision.
If a word or passage has been intentionally removed, it is a redaction.