Senior Freeze base year
The tax year used as the comparison point for an eligible Senior Freeze applicant's future property-tax reimbursement calculation.
New Jersey example
The useful number is the eligible base-year tax amount, not simply the homeowner's current age or current bill.
Official source
https://www.nj.gov/treasury/taxation/ptr/
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.