The spelling isn’t the only thing that is in doubt, although of the four versions in the title the first and the fourth seem to be preferred by some. Ask.com says “The middle of a target is called a bullseye because…archers would practice by shooting at a bull’s skull. They would practice until the hit the eye of the bull. Allexperts.com disagrees. That site has a complicated description that the origin is unclear, but not related to archery. I’ll give a short version of their theory, and that it is related to the word “center,” and “is in reference to glass—a thick boss or prominence in the center of a glass pane.” Regardless, we use the term to mean the center of a target or to describe that we have achieved exactly the goal we set out to meet.