Being "open," or non-mono, is the larger heading. Polyamory is a subset of non-monogamy.
Other categories under the non-mono heading are "don't ask, don't tell" relationships, and swinging. In ancient times (and today in neo-pagan groups), married couples could also take part in a non-mono experience when worshiping a god or goddess in a temple with a "holy one" (priest or priestess representative of certain gods or goddesses), or in orgies at Beltane and other holy days.
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
Mags, F, 60, poly-dating, loving and living with
miss pixi, F, 38
Punk, 42, M (dating since Oct 2015)