The Defense of Marriage Act (DOMA) (Pub.L. 104–199, 110 Stat. 2419, enacted September 21, 1996, 1 U.S.C. § 7 and 28 U.S.C. § 1738C) was a United States federal law that, prior to being ruled unconstitutional, defined marriage for federal purposes as the union of one man and one woman, and allowed states to refuse to recognize same-sex marriages granted under the laws of other states. Until Section 3 of the Act was struck down in 2013 (United States v. Windsor), DOMA, in conjunction with other statutes, had barred same-sex married couples from being recognized as "spouses" for purposes of federal laws, effectively barring them from receiving federal marriage benefits. DOMA's passage did not prevent individual states from recognizing same-sex marriage, but it imposed constraints on the benefits received by all legally married same-sex couples.
Initially introduced in May 1996, DOMA passed both houses of Congress by large, veto-proof majorities and was signed into law by President Bill Clinton in September 1996. By defining "spouse" and its related terms to signify a heterosexual couple in a recognized marriage, Section 3 codified non-recognition of same-sex marriages for all federal purposes, including insurance benefits for government employees, social security survivors' benefits, immigration, bankruptcy, and the filing of joint tax returns, as well as excluding same-sex spouses from the scope of laws protecting families of federal officers (18 U. S. C. §115), laws evaluating financial aid eligibility, and federal ethics laws applicable to opposite-sex spouses.
In United States v. Windsor (2013), the U.S. Supreme Court declared Section 3 of DOMA unconstitutional under the Due Process Clause of the Fifth Amendment. Obergefell v. Hodges (2015) struck down the act's provisions disallowing same-sex marriages to be performed under federal jurisdiction.
This summary is from Wikipedia.