A loving, committed relationship between 3 people. All 3 people are equal in the relationship and typically sexually intimate with both of the other two. Like a marriage, but with 3 people.
Most often found between 3 gay men, but could also be (in order of likelihood) 2 men and 1 woman, 2 women and 1 man or 3 women.
A Triad is a partnership of equals and does not have the same stigmas associated with it as does polygamy which is sometimes (not always) more about a man married to and/or controlling multiple women who are not involved with each other sexually.
"Brian and Duane have been together for about 6 months, but Richard just joined them to become an official triad."
David and Cynthia have been married for a while, but Mike just moved in and they are now a triad.