A group bound together, where all parties are primary members, engaged in sex only with those within the group.
A group, that has more than just a sexual relationship, which has multiple members, where all parties engage in sexual relations with each other, but not with any person outside the group.
The 'marriage' between several consenting adults, into a relationship where the stated terms define that there are no other sexual partners, other than those within the 'marriage'.
In most respects, it is similar to a Polyamorous group marriage, or if one wanted to take it further, to a Monogamous marriage but with multiple partners.
Not to be confused with a menage a trois. This is about where all partners are emotionally attached to each other, and bound together. This is not an 'open' marriage by definition, as the sexual contact is only between those within the group.
Members agree to be faithful to those within the group, by not seeking any sexual encounter outside, thus it is considered a 'closed' relationship by definition.
Noteworthy: Some believe that it is impossible to define who one will find an emotional and sexual attraction to, with such constraints as contained within a Polyfidelity Relationship.
They also claim it is difficult to find enough participants, to meet the stringent requirements of Polyfidelity.