Property tax appeal
A request to your county board of taxation to change your property's assessed value. You can appeal the assessment, but not the tax bill itself, because the taxes come from local budgets.
New Jersey example
In the state's appeal guide example, a home worth $100,000 is assessed at $110,000 in a town whose average ratio is 95.41%. That assessment is 110% of value, above the county's 100% level, so the board applies the average ratio: $100,000 x 95.41% = $95,410. The burden of proof is on the taxpayer, so you need evidence.
Official source
A Guide to Tax Appeal Hearings (NJ Division of Taxation)
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.