Stands for a photo developed and sold on the same day it was taken. (The term is commonly used by race day photographers. ZDP is commonly used but not limited to the subjects of running, biking, swimming, football, basketball, tennis, golf, race car, or fishing photography.)
Can be used like, "Hey, do you have any zero day photos of this event? I want to get mine today!" or "Do you know where I can get some zero day photos of this event? I want some photos that were taken of me from this race but I want them now!" Zero day photo is sometimes used in the form of an acronym like, "Hey, you have any ZDP's of this event?"