Well first of all, strength is a spectrum, on one end you have your max force development (powerlifters, oly lifters), and on the other end, extended low-level force development (ultramarathon types).
So to me, strength is anything that lies on that spectrum.
But mostly to me its a mental choice. I love Bernarr MacFadden's quote, "weakness is a crime" that simple quote and his justification explains pretty much my outlook on life. Weakness is a choice, strength is a privilege.
Love that you started this thread btw, I could go on for hours, Im a huge physical culture nut/historian.
EDIT: This is just my opinion and the ideology I've latched onto, not saying its right.