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.
It sounds like you're talking about lacunae.
Words deliberately left out or blanked for security reasons would be redacted
A caret mark (^) used to note an omission but generally where something is to be inserted. An eclipsis denotes "omission or suppression of parts of words or sentences [syn: ellipsis]". Note: 'caret' is Latin for missing
Haplography n. - accidental omission of letters, words or lines in copying
Interesting that this sticks:
Haplography is the act of writing once what should be written twice. For example, the English word "idolatry", the worship of idols, comes from the Greek "eidololatreia", but one syllable has been lost through haplography. Other examples are "endontics" for endodontics, "tillate" for titillate.