Something that serves as indisputable evidence or proof, especially of a crime.

Here is an example used in everyday language from CNN.com on Feb 6, 2002:

"Maybe there was no proof before, but there is now; a secret memo - personally handed to [U.S. Vice-President Dick] Cheney by Ken Lay [ex-Enron chairman and CEO], which helps explain why the White House is so skittish about Enron and why Cheney and [U.S. President George W.] Bush stubbornly refuse to release the records of those energy task force meetings. The memo was obtained by the San Francisco Chronicle and reported exclusively there last week. This is the Enron smoking gun."