It's a really good examination of the concept Susan. And it is quite possible that you've hit the nail on the head with your assertion that it's contextual.

I see it as you do, that anyone can do leadership at any given time, and its possibly the frequency of leadership acts that causes others to begin to attribute the word 'leader' to another. But how any one person becomes defined by their unique collection of leadership acts is surely down to lots of different variables, (take a Nelson Mandela vs. a Jeffrey Immelt, for example).

Obviously, any definition needs to include leaders, followers, relationship, influence, purpose, movement, change, etc, and I guess the fact that the elements it has to have causes people to try to define it. But it's possible that the fact we can't put a finger on the 'defining' definition actually makes it more dynamic and beautiful (at least when ignited around worthy causes).

I like Joseph Rost's (1993) definition, "An influence relationship among leaders and followers who intend real changes and outcomes that reflect their shared purpose", though I accept it is not the definition that eclipses all others. It just works for me and I find it keeps me both communicative and focused.