If you are reimbursed under an accountable plan, then reimbursements may be tax free. For example, if reimbursements for your mileage exceed the IRS standard mileage rate, then only amounts up to the rate are tax free; excess reimbursements are taxable.
An amount taken from income as a prepayment of an individual’s tax liability for the year. In the case of wages, the employer withholds part of every wage payment. Backup withholding from dividend or interest income is required if you do not provide the payer with a correct taxpayer identification number. Withholding on pensions and IRAs is automatic unless you elect to waive withholding.