Bigamy is the legal term for being married to a second spouse else when one is already wed to another. It is considered illegal in the United States, and the second marriage is considered void.
In the United States, people can legally marry only one person at a time. Although marriage is regulated at the state level, and the law does differ somewhat from locale to locale, as of 2015, all 50 states are unanimous in confining legal marriage to pairs, instead of triads or larger groups. If one wishes to legally marry a second spouse, the first marriage must be terminated, via divorce, annulment or death.
Polygamy is distinguished from bigamy by how the plural marriage was performed. While bigamy involves legally marrying another spouse in compliance with all other state laws, polygamy involves having multiple spouses, who may be bonded by a spiritual marriage ceremony or common law marriage. A bigamous marriage involves a state marriage license, while a polygamous marriage does not.