The area directly in front of a urinal in the men's room where there is chronically a little puddle of foul urine. This is caused by the little drops at the beginning and end of the flow that don't make it into the urinal because the guy doesn't want to stick his dick right into the dirty thing. However, the drop zone is an accepted part of the male culture, as is the default three inches from the actual urinal that you must stand in order not to pick up some disease from virtually humping the thing to avoid an addition to the drop zone. (another unfortunate result of standing too close to a urinal is splashback when your piss hits the vertical wall and deflects back at you in a hail of little drops)
"Woah woah, son. Always watch out for the drop zone when you pee in this thing. Keep your legs spread a little and your feet in a slighly outward angle, and you won't step in it."