long shot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An entry, as in a horserace, with only a slight chance of winning.
• noun A bet made at great odds.
• noun A venture that offers a great reward if successful but has very little chance of success.
• noun A photograph or a movie shot in which the subject appears small or distant in relation to the rest of the image.
• idiom (by a long shot) To any extent; at all. Usually used in negative sentences.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic, nautical Something unlikely; something that has little chance of happening or working. The term arose from the accuracy of early ship guns, which were effective only at close range and unlikely to hit the mark at any great distance.
• noun film A master shot, the primary wide shot of a scene into which the closeups will be edited later.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a contestant that is unlikely to win
• noun a venture that involves great risk but promises great rewards