Form 709 Gift Tax, United States Gift (and Generation-Skipping Transfer) Tax Return
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 or not the donor intends the transfer to be a gift.
The gift tax applies to the transfer by gift of any type of 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.
Source page: https://www.irs.gov/forms-pubs/about-form-709