common-law marriage

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

• noun A form of marriage, available in some jurisdictions, that may be established by meeting certain legal requirements such as declaring the intent to be regarded as married and cohabiting, rather than as a result of obtaining an official license.

from The Century Dictionary.

• noun See marriage.

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

• noun A marriage based on the duration of cohabitation rather than formal ceremony.

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

• noun a marriage relationship created by agreement and cohabitation rather than by ceremony