I'll try to paraphrase the best definition I've come across from, of all places, a Harvard b-school prof.: "An enetrepreneur is someone who pursues (as opposed to talks about or plans) a perceived strategic opportunity (perception being the entrepreneurial advantage) regardless of tactical ability (money, position, credibility, skill set, connections, resume, background, etc.).
A bias for action, superior "a priori" perception of potential payoffs, and a high tolerance for bluffing and "faking it till you make it."
I have found no better practical explanation that covers the whole spectrum.