civil union

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A legally sanctioned relationship between two people, especially of the same sex, having many of the rights and responsibilities of marriage.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A legal union similar to marriage, established to allow similar rights to same-sex couples, and in some jurisdictions opposite-sex couples, as partners in traditional marriages have.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a voluntary union for life (or until divorce) of adult parties of the same sex