A federal tax that must be paid by people who hire household help (a babysitter, maid, gardener, etc.) and pay them a total of more than a specified threshold amount during the tax year. The reason the IRS charges the nanny tax is because it considers an ongoing household helper to be the taxpayer's employee. As such, the taxpayer becomes an employer and must pay Social Security, Medicare and federal unemployment taxes on the wages paid to that employee. There may be state-level nanny taxes as well.