Some people just say OSO, other significant other. Of course, there is a poster here who is a... secondary, who was thinking of herself as the Other Woman, which obviously has mono, negative connotations. But somehow "other significant other" sounds OK.
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
There's no lying in polyamory!
I'm a 58 year old woman with 2 partners:
miss pixi, my live-in gf, 36 (together since Jan '09)
Ginger, bf, 61, married, lives nearby (together since Jan '12)