People in general, but I'm comfortable applying the idea to baseball players and other professional athletes.
Yes, to be an MLB player you have to be mentally strong and you have to overcome adversity. But I don't think it's as black and white as, you're mentally strong as a person or you're not. Your mental strength can be different at different points in time just like your other skills are. Mental strength might be one of the skills that is still developing in some players, and it might develop in different ways depending on what environment they're in, including what happens to them on the field.