A function you pass to another function which runs the function you passed to the function you are calling, often this function that calls the function you passed to the function supplies your function with parameters created from results of calculations within the function you passed your function to.
A dead drop is a location used to secretly pass items between two people, without requiring them to meet.
Spies have been known to use dead drops, using various techniques to hide the items and to signal that the drop has been made. Drug dealers use this as well.
A common place is to duct tape the item to the underside of a toilet cover in a public restroom.
A signal is sometimes used as well to indicate there is something to pickup, like a pile of stones in a certain formation. After checking the drop the signal is removed.
"The dead drop is the restroom toilet at the Gulf station at the corner of Commercial Ave. and Georges Rd."