Being married to more than one person.
If just two wives or husbands, can be referred to as bigamy.
Canada's Criminal Code:
"293(1). Every one who practises or enters into or in any manner agrees or consents to practise or enter into any form of polygamy, or any kind of conjugal union with more than one person at the same time, whether or not it is by law recognized as a binding form of marriage, or celebrates, assists or is a party to a rite, ceremony, contract or consent that purports to sanction (such) a relationship .... is guilty of an indictable offence and liable to imprisonment for a term not exceeding five years."
In 1862, the US government passed an anti-bigamy law which quashed Utah's laws which at the time, established, supported and maintained polygamy.
From the present Constitution of the US State of Utah, ARticle 24, ¶2:
"All laws of the Territory of Utah now in force, not repugnant to this Constitution, shall remain in force until they expire by their own limitations, or are altered or repealed by the Legislature. The act of the Governor and Legislative Assembly of the Territory of Utah, entitled, "An Act to punish polygamy and other kindred offenses," approved February 4th, A.D. 1892, in so far as the same defines and imposes penalties for polygamy, is hereby declared to be in force in the State of Utah."