One that has multiple relationships with others. Rarely do the others have relationships with each other. Some call it responsible non-monogamy. Also called: group marriage, non-monogamy, open marriage.
There are some sub groupings:
Polyfidelity- Group of people committed to each other and do not have sex or romantic ties outside of the commitment group.
Polysexual- Pretty much swingers, having sex and no commitment with others, often done in groups.