I use the word "friend." It's the best description I can come up with. In my personal philosophy, there's very little difference between what one usually thinks of as friendship and romance. To me, friendship is the strongest bond there is. Romance seems to me to be too often more about ideals and expectations, whereas friendship is real.
Two friends of mine, I'll call them Jeff and Krystal, started dating when they first met. They decided shortly after that that it wasn't working for them, and decided to be friends. 14 years later, they are best friends, and love each other a lot. They even have sex once in a while. In the time they've known each other, they've has many arguments. However, they always end up working things out. Had they continued to pursue a romantic relationship, they would have gone their separate ways long ago.
A few years ago, Jeff set Krystal and I on a blind date. It didn't work out because we both thought the other wasn't interested, but we did become friends. Recently, we both discovered that the other was polyamorous, and that we were, in fact, interested in each other. We're now talking about becoming "better friends," as I would phrase it. There's also plans of introducing her to another couple I'm involved with (assuming those two and I can work out some issues I started a thread in "New to Polyamory" about. Which, slow progress is being made.) Now that Krystal and I have known each other for a while, pursuing this kind of relationship now seems like it'll work out a lot better that it would if we forced it to happen when we first met.
Using the word "friend" is also adaptable to most situations, and prevents the possibility of accidentally revealing the situation to those that aren't in on it. For instance, my father knows about my "new friend," but not that she's my best friend's girlfriend. If he figures out that they're together, it's an ambiguous enough term that I can pretend nothing is going on. I'll tell him eventually, but not until I have to. However, I can also put emphasis on the word "friend" when in situations where I want to communicate that a given "friend" and I are "totally doin' it," as it were.