For federal tax purposes, a spouse is “an individual who is lawfully married to another individual.” Lawfully married means that the marriage is “recognized by the state, possession, or territory of the United States in which the marriage is entered into.” People married in a foreign jurisdiction are recognized as married for federal tax purposes “if the relationship would be recognized as marriage under the laws of at least one state, possession, or territory of the United States.” Both of those definitions would apply equally to opposite-sex and same-sex marriages. The law does not recognize as spouses individuals in a registered domestic partnership, civil union, or other similar formal relationship “not denominated as a marriage under the law of the state, possession, or territory of the United States where such relationship was entered into.”