Federal Estate Tax, Marital Deduction
Prior to the calculation of the federal estate tax, the full value of all property that passes to a surviving spouse who is an American citizen is deducted from the deceased spouse's gross estate. This allows that property to pass to the surviving spouse free of federal estate tax.

This is also known as the "Unlimited Marital Deduction" because all of the property received by that surviving spouse is free of the federal estate tax.

See: Federal Estate Tax, Generally; Federal Estate Tax, Exclusion Amount