Faithlessness, disloyalty, or treachery.
Example:
While working for the CIA he became a double agent for another country, and it seems he paid a high price for his perfidy.
The Latin phrase per fidem decipere, meaning "to betray the trust of," may have been the original source of perfidus, from which perfidy comes. The most famously perfidious figure in U.S. history is probably Benedict Arnold, the American army officer in the Revolutionary War who plotted with the British to surrender West Point to them -- an act that made his name and epithet for traitor.