a claim by a taxpayer [to the IRS] that he/she is entitled to a refund of all (or part) of the taxes that he/she paid [in earlier years]. Such a claim must be made in writing within a specified time from the filing date of the return to which it relates. Plus, it is a necessary prerequisite to any allowance of a refund by the IRS (or to any suit by the taxpayer against the IRS – for disallowance of a claimed refund).