26 C.F.R. § 301.7701-18 : Internal Revenue — INTERNAL REVENUE SERVICE, DEPARTMENT OF THE TREASURY — PROCEDURE AND ADMINISTRATION — PROCEDURE AND ADMINISTRATION —DEFINITIONS —18 - Definitions; spouse, husband and wife, husband, wife, marriage

(a) In general. For federal tax purposes, the terms spouse, husband, and wife mean an individual lawfully married to another individual. The term husband and wife means two individuals lawfully married to each other.
(b) Persons who are lawfully married for federal tax purposes-
(1) In general. Except as provided in paragraph (b)(2) of this section regarding marriages entered into under the laws of a foreign jurisdiction, a marriage of two individuals is recognized for federal tax purposes if the marriage is recognized by the state, possession, or territory of the United States in which the marriage is entered into, regardless of domicile.
(2) Foreign marriages. Two individuals who enter into a relationship denominated as marriage under the laws of 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, regardless of domicile.
(c) Persons who are not lawfully married for federal tax purposes. The terms spouse, husband, and wife do not include individuals who have entered into 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, regardless of domicile. The term husband and wife does not include couples who have entered into such a formal relationship, and the term marriage does not include such formal relationships.
(d) Applicability date. The rules of this section apply to taxable years ending on or after September 2, 2016.

26 C.F.R. §301.7701-18

T.D. 9785, 81 FR 60616, 9/2/2016