Equanimity

The capacity to be proactive rather than reactive, to not be "thrown" easily, to be a present witness to what is going on around and within oneself.

I'd heard the term before, but had a deeper understanding of it and took it on as a quality to nurture in myself and seek out in others when i first did a 10-day meditation course with dhamma dot org. It is a central purpose of the vipassana meditation they teach, and the only measure of progress in it.