"Ability to produce a positive outcome through near perfect performance."
– Clifton StrengthsFinder
I think this one stuck with me, because in being so clinical it broke down the idea of strength in a way I have never heard before... essentially that strength about how you perform. I think that that is more intuitive when thinking about physical strength, but I don't know that I'd mapped that over totally to mental strength. Of course what "perfect" and "performance" mean depend on context and desires, but I found it helpful to reframe a statement like "she is a strong person" as "she's doing the thing" ...what ever that thing might be. That strength is about performing at your best.