Putting Zero in the title of a work that is meant to be the beginning, usually a prequel.
Do We Have This One?? Well math says that 0 comes before 1, so putting "Zero" in the title of a work can be a good shorthand that a work is a prequel, prologue, or even a remake of a first chapter. Some comic book origin stories are shown in issues numbered 0. Of course if "Zero" is in the title for any other reason, it doesn't count (as it wouldn't be the beginning). So works like Mega Man Zero and Phantasy Star Zero wouldn't be an example. A Sub-Trope of Numbered Sequels. Compare Unusual Chapter Numbers, Year Zero.