Also known as the Landolt ring. A ring with a small gap somewhere on it, giving the ring the look of the letter C.
This is utilized in a form of vision examination: Landolt Cs are arranged in rows with varying gap positions (usually the four cardinal directions, or the 45-degree placements between them), with each row getting progressively smaller. The person being examined must attempt to correctly state where each gap on the Landolt Cs can be found until a certain number of errors are made.
This tag implicates eye_chart (learn more).