State And Local Taxes, which used to be fully deductible on your federal income tax, but are now capped at $10K. Any tax over that amount is not tax deductible on your federal 1040.
(My SALT luckily came in at just $10,020 last year, and I'm still early in a mortgage and have a lot of interest to deduct too. Now that the standard deduction for individual filers has been raised to $12,000 I'm coming out ahead of that, but, as the curve changes I'm going to have to keep an eye on where things go from here)