A gift tax is a tax imposed on the transfer of ownership of property during the giver's life. The United States Internal Revenue Service says that a gift is "Any transfer to an individual, either directly or indirectly, where full compensation (measured in money or money's worth) is not received in return."

When a taxable gift in the form of cash, stocks, real estate, or other tangible or intangible property is made, the tax is usually imposed on the donor (the giver) unless there is a retention of an interest which delays completion of the gift. A transfer is "completely gratuitous" when the donor receives nothing of value in exchange for the given property. A transfer is "gratuitous in part" when the donor receives some value but the value of the property received by the donor is substantially less than the value of the property given by the donor. In this case, the amount of the gift is the difference.

In the United States, the gift tax is governed by Chapter 12, Subtitle B of the Internal Revenue Code. The tax is imposed by section 2501 of the Code. For the purposes of taxable income, courts have defined a "gift" as the proceeds from a "detached and disinterested generosity." Gifts are often given out of "affection, respect, admiration, charity or like impulses."

Generally, if an interest in property is transferred during the giver's lifetime (often called an inter vivos gift), then the gift or transfer would not be subject to the estate tax. In 1976, Congress unified the gift and estate tax regimes, thereby limiting the giver's ability to circumvent the estate tax by giving during his or her lifetime. Some differences between estate and gift taxes remain, such as the effective tax rate, the amount of the credit available against tax, and the basis of the received property.

There are also types of gifts which will be included in a person's estate, such as certain gifts made within the three-year window before death and gifts in which the donor retains an interest such as gifts of remainder interests that are not either qualified remainder trusts or charitable remainder trusts. The remainder interest gift tax rules impose the tax on the transfer of the entire value of the trust by assigning a zero value to the interest retained by the donor.