While it may not be a real word, it's an idea that seems to be of concern in poly relationships. The idea being that, while you share your partner(s) with others, it still feels good to have a few things that are symbolic of the relationship that is between you, not inclusive of others.
Here's an example from our household: A couple years ago, I gave some cologne to PG that I REALLY like. A lot. In a way that makes me want to rip his clothes off and have my wicked way with him. It's also been discontinued. I've requested that he reserve that specific cologne for dates with me, because it's special, and in limited supply.
"Oh, but you're controlling your partner, and depriving someone else from enjoying that cologne on him." This is one of those cases where I think that's ridiculous. If I were saying that he couldn't wear any cologne with other people at all, perhaps so. It wouldn't be a deal-breaker- relationship-severing transgression if he decided to wear it elsewhere, but I would feel disappointed and hurt.
Let's take another example: S and I had a date that was extremely special and emotionally resonant early on in our relationship. We've agreed that the particular set of venues involved are ones that we will be keeping between the two of us, as a way to honor our bond and renew it on occasion. In the same fashion, I won't be taking S to the hotel that PG and I honeymooned at. At best, it would be tacky and in poor taste.
While prevailing opinion is split as to whether having things that are "special" enhance a bond, or create an artificial sense of exclusivity within poly relationships, it seems to me that it's just another way to acknowledge that each one of my partners is unique, each relationship is it's own entity, and there can be sacred space created within a larger poly structure that doesn't negatively impact the whole.