Federal law which defines ‘marriage’ at the federal level.
Under the Defense of Marriage Act (DOMA):
The word “marriage” means only a legal union between one man and one woman as husband and wife, and
The word “spouse” refers only to a person of the opposite sex who is a husband or a wife.
IRC -1 § 7 , IRC-28 § 1738C, Public Law No. 104-199,
Additional Helpful Information
Under DOMA, no State, territory, or possession of the United States, or Indian tribe, is required to recognize marriage between individuals who happen to be of the same sex, if the marriage occurred in another State.
If a State does not recognize same-sex marriage, it is not required to allow benefits that are only offered to married couples, who are of the same sex.