IRS: Gift Taxes
The gift tax is a tax on the transfer of property by one individual to another while receiving nothing, or less than full value, in return. The tax applies whether the donor intends the transfer to be a gift or not.
The gift tax applies to the transfer by gift of any property. You make a gift if you give property (including money), or the use of or income from property, without expecting to receive something of at least equal value in return. If you sell something at less than its full value or if you make an interest-free or reduced-interest loan, you may be making a gift.