Court to hear appeal over federal gay marriage law NEW YORK (Reuters) - The next battle over a U.S. law that defines marriage as a union between a man and a woman will be waged in a federal appeals court in New York on Thursday. The 2nd U.S. Circuit Court of Appeals will hear arguments in the case of Edith Windsor, an 83-year-old woman who says the Defense of Marriage Act discriminates against gay couples in violation of the U.S. Constitution. Windsor's lawsuit is one of numerous challenges to the Defense of Marriage Act winding their way through U.S. courts. Appeals in several cases are pending before the Supreme Court, which could choose to take up the issue in its upcoming term that starts on Monday. Six states have legalized same-sex marriage, including New York in 2011. Because of the Defense of Marriage Act, which was passed in 1996, federal law and government programs do not recognize those marriages. The lawsuit was filed on behalf of Windsor by the American Civil Liberties Union in federal court in New York. Windsor is a former IBM computer programmer who married Thea Clara Spyer in Toronto, Canada, in 2007. The two were engaged in 1967. Spyer died in 2009 after a decades-long battle with multiple sclerosis, leaving all of her property to Windsor. Because the marriage was not recognized under federal law, Windsor had to pay more than $363,000 in federal estate taxes, according to her lawsuit. Windsor's attorneys argue that the act violates the 14th Amendment of the U.S. Constitution, which guarantees equal protection under the law. In June, a federal district judge in New York ruled for Windsor, finding that a central provision of the Defense of Marriage Act discriminates against married same-sex couples. Federal courts in Massachusetts, California and Connecticut have also found the law unconstitutional. The Obama administration said last year it considered the law unconstitutional and would no longer defend it. Instead, a group appointed by the Republican majority in the U.S. House of Representatives is defending the law in courts across the country. The group appealed the district court's decision in the Windsor case to the 2nd Circuit, which expedited the appeal due to Windsor's poor health. She suffers from a serious heart condition that could end her life before the case is resolved. Windsor has asked the Supreme Court to review her case before the 2nd Circuit reaches a decision. Windsor's lawyer, Roberta Kaplan of Paul, Weiss, Rifkind, Wharton & Garrison, said Windsor was a hero for continuing to fight on behalf of others in her position. Paul Clement of Bancroft, who will defend the Defense of Marriage Act before the 2nd Circuit, did not respond to a request for comment. Appeals in four cases that challenge the Defense of Marriage Act are pending before the Supreme Court, which has not yet decided whether it will take up the issue. The Justice Department has filed petitions in all four cases, asking the high court to review the constitutionality of the law's definition of marriage. The case is Windsor v. USA et al, 2nd U.S. Circuit Court of Appeals, No. For Windsor: Roberta Kaplan of Paul, Weiss, Rifkind, Wharton & Garrison. For the Bipartisan Legal Advisory Group: Paul Clement of Bancroft. For the Justice Department: Stuart Delery. Editing by Eddie Evans and Vicki Allen