Hang on, folks. Here's another definition that goes beyond what was decided in U.S. v. Windsor. The Employee Benefits Security Administration (EBSA) has a new Technical Release [here] saying that ERISA and the Internal Revenue Code will be read so the term "spouse" and "marriage" will refer to any individuals who are lawfully married under any state law, including individuals married to a person of the same sex who were legally married in a state that recognizes such marriages, but who are domiciled in a state that does not recognize such marriages.
United States v. Windsor (US Supreme Court 2013) didn't go that far because it applied to a couple whose out-of-state marriage was recognized by the state where they lived. And that fact was important in the Court's analysis. OK, That won't matter when the government interprets ERISA.
Hat Tip: Paul Secunda, Workplace Prof Blog