Good article discussing the "portability" aspect of the estate (death) tax.
The “portability election” refers to the right of a surviving spouse to claim the unused portion of the federal estate tax exemption of their deceased spouse and add it to the balance of their own exemption. Since in 2015 the federal estate tax exemption is $5.43 million per person (the exemption changes every year since it is indexed for inflation), this means that a married couple can potentially pass on $10.68 million to their heirs free from federal estate taxes.
To properly make the portability election, the surviving spouse must timely file a federal estate tax return, known as the “United States Estate (and Generation-Skipping Transfer) Tax Return,” or “Form 706” for short.