Omitted assessment
An assessment used when taxable property or an improvement should have been assessed but was omitted from the regular roll.
New Jersey example
An omitted improvement can create a later bill even though it did not appear on the original annual assessment notice.
Official source
https://www.nj.gov/treasury/taxation/lpt/localtax.shtml
This is a plain-English summary. Rules and dollar amounts change, so check the official source or a professional before you file or sign anything.