For federal estate tax purposes, the terms “spouse,” “husband,” and “wife” includes individuals of the same sex who were lawfully married. However, the terms “spouse,” “husband,” and “wife” do not include individuals (whether of the opposite sex or the same sex) who have entered into a registered domestic partnership, civil union, or other similar formal relationship recognized under state law that is not denominated as a marriage under the laws of that state, and the term “marriage” does not include such formal relationships.
All property that is included in the gross estate and passes to a surviving spouse is eligible for the marital deduction. The property must pass "outright." In some cases, certain life estates and certain marital trusts (for example, a so-called QTIP trust) also qualify for the marital deduction.