"Drop" is intelligence parlance for the location at which an agent passes information to another, or the act of passing that information—as in "making a drop." In a live drop, the two individuals actually meet. Given the dangers of this, it is more common to employ a "dead drop." The latter term refers to a prearranged spot at which one party passes information to another without actually meeting. Often a dead drop—a term that again refers both to the place and the act—also involves the transfer of money, as when a double agent leaves information for a handler, and the handler returns the favor with cash payment.
It so happens that the most commonly cited examples of drops and dead drops involved agents working for the Soviet bloc during the Cold War. This is probably the case because, for obvious reasons, Western intelligence agencies are not as likely to reveal the methods employed by their own agents.
One oft-cited example is that of John Walker, who passed $1 million of United States Navy secrets to the Soviets before the Federal Bureau of Investigation (FBI) finally caught up with him in 1985. In making his drops, Walker used a garbage bag containing bits of recognizable trash—but nothing that would smell strongly and attract animals—along with documents and other important materials. His KGB handler would in turn leave another bag containing money.
In the same year the FBI caught Walker, the Soviets recruited the FBI's own Robert Hanssen, who accumulated $1.4 million for betraying his country before the authorities caught him in February 2001. At the time of his arrest, Hanssen was making a dead drop under a footbridge at Foxstone Park in Vienna, Virginia.
█ FURTHER READING:
Nash, Jay Robert. Spies: A Narrative Encyclopedia of Dirty Deeds and Double Dealing from Biblical Times to Today. New York: M. Evans, 1997.
Polmar, Norman, and Thomas B. Allen. Spy Book: The Encyclopedia of Espionage. New York: Random House, 1998.
"Traitorous Actions": FBI Agent Charged With Spying for Moscow. < http://abcnews.go.com/sections/us/DailyNews/FBIarrest010220.html > (February 1, 2003).