a state, condition or instance of serious or continued difficulty or adverse fortune.
Thanks to Merriam-Webster for that definition. Building a start-up has a lot of this. I would more categorize it, however, as a feeling of, "Damn it!" That's a pretty good phrase for moments in the start-up world. You have great ideas, and lack the cash to implement them. You have the perfect product, but not enough partners to make a significant impact yet. You have this internal drive, but you still can't get every door opened.
The reaction to adversity is the difference between the entrepreneur and the wannabe. The wannabe feels the adversity and complains (oft times loudly) at the unfairness of it all. The entrepreneur recognizes the adversity, then gets back on the phones, back to the networking, rewrites the code, stays up late, re-examines their position, focuses their attention, gets on the phone again...lather, rinse, repeat.
The reason for this is simple: Vision. The true entrepreneur (which can only be measured by spirit, and not by money) can see clearly that, while there may (will!) be adversity ahead, that nothing about it is permanent. Go back and read the definition for yourself. It is nothing more than a encapsulation of a specific moment in time. It also says nothing about defining you, only about defining that specific moment in time.
It is not the adversity that defines us, but how we indeed react to it. That is why some of us are entrepreneurs and some of us are not. It is not about the success or failure of the enterprise, but what we learned from the experience and how we audited ourselves. And, in the event of "failure", how long it took us to get back up and start again - with the wisdom that our last effort taught us.
Being an entrepreneur is not for everyone, but it is for me. I am not smarter, better or tougher than anyone. Far from it. I simply recognize that this rough spot is followed by smooth water - I just have to know when to keep paddling the boat or when to let it sink and swim the rest of the way. Or, if I'm really smart, build a better boat.