According to Harvard Business School professor Howard Stevenson in Breakthrough Entrepreneurship written by Jon Burgstone and Bill Murphy, Jr.
"Entrepreneurship is the pursuit of opportunity without regard to resources currently controlled."
Said another way by Burgstone:
"Every time you want to make any important decision, there are two possible courses of action. You can look at the array of choices that present themselves, pick the best available option and try to make it fit. Or, you can do what the true entrepreneur does: Figure out the best conceivable option and then make it available."