Comparable sales (comps)
Sales of properties similar to yours that you use as evidence of your property's market value in an appeal. Sale prices count as evidence; other properties' assessments do not.
New Jersey example
The state's appeal guide says to attach 3 to 5 comparable sales to your appeal when you file, and the assessor and county board must get copies at least 7 days before the hearing. Use sales from before the October 1 assessment date, and add photos of your home and the comps.
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.