I have to admit, seeing so many people choose, "I always call him/her my son/daughter, without the step" is enlightening! I expected that to be a minority choice, not a tie with the answer I chose.
However, from the comments it seems like "son/daughter" is more common for those of you whose biological counterpart is not in the picture. Incorrigible referred to her kids being devastated when her DH used "step", feeling that "NO ONE wanted to be their Dad". Well, that's easy to understand, if there is no other Dad!
Aricha's comment (as usual!) really has me thinking. You have a long-distance situation like I do and feel like there's no reason to clarify the "step" since no one in your community knows your DSD's mom. I realized that a significant part of my instinct to use "step" is, perhaps, place-holding for DSS's mom. Very few people in our community know her, either, even though she does come here to visit a couple times a year. But she makes no effort to meet DSS's teachers, friends, their parents, etc. It's common knowledge that she looks at his whole life here as a temporary situation, until he turns 14, testifies that he wants to "go home" to her, moves back to California and never returns. I guess I keep thinking that, as time goes on (or, if DH winds up keeping custody while DSS is in high school), that DSS's mom might start behaving in ways I consider healthier, like getting involved with his education and friendships or even moving back here. It's hard for me to imagine a mother never breaking down and doing those things. Regardless of all the issues of pride and awkwardness, that's her only child!
So, in a way, I guess I'm looking at this as a temporary situation, too: I don't assume DSS living with us is temporary, but I hold out the possibility that me doing the vast majority of the "mothering" jobs may be temporary. Maybe I remain aware of that out of respect for DSS's relationship with his Mom. Maybe it has more to do with me not letting myself get 100% attached to my current role, such that if it changes, I won't grieve quite the same way as if I were "demoted" in my role with my biological children. And, just as I've put that into words, I realize how silly it is. If my current role with DSS changes (after 4 years getting used to it), nothing could shield me from grieving over it!
Anyway, DSS and I discussed this. I told him my instinct has been to use the "step", not because I love him less or feel like he's in any way a lesser part of the family, but simply because I know he has a mother and I don't want to sound like I'm trying to replace her. But I explained that plenty of other people look at it differently and don't use "step", so if he'd like me to do it differently I'd be happy to. He didn't address the idea of whether or not he (or his mother) would worry about me sounding like I'm replacing her. But he very quickly piped up that I am his step-Mom; that's what he calls me; and he doesn't think there's anything wrong with it, or feel bad about it. So, we're good!
I have to be honest, as I've thought about it, despite my positive relationship with my older sons' step-Mom, I wouldn't want her calling herself their mother. I know she includes them in phrases like "our sons" (referring to her DH's sons and their sons together, without distinguishing who's whose). Of course, I do that with DSS, too. But for her to run into a friend while shopping with my sons and introduce them as "my sons"? Or for her to meet their teachers and introduce herself as "their Mom"? That would rub me the wrong way. And, to be clear, I understand the value of my kids' relationship with her to the extent that I offer her parenting time, if my ex has extended work conflicts and can't see them for awhile. But I still don't see her as a completely equal parent.