Appraisal
An independent expert's opinion of a home's fair market value, meaning the price a willing buyer and seller would agree to. It is separate from your tax assessment, the assessor's value as of October 1 that your property tax is based on.
New Jersey example
Made-up example: a homeowner who thinks the assessment is too high appeals to the Tax Court using an independent appraisal. A copy must be submitted no later than 20 days before trial, and the appraiser must testify, or the evidence could be dismissed. Assessments on other homes cannot be used to prove value.
Official source
Tax Court Self-Help | NJ Courts
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.