The word meridian describes a gigantic imaginary circle that runs north and south on the earth's surface, from the North Pole to the South Pole. If you stood on the meridian at the North Pole, you would be at the earth's northernmost point.
Meridian can also refer to the highest stage of development. The meridian of your chess career may have been when you took out the champ from a rival school. Used as an adjective, meridian can describe something that's at the best stage of development. Your favorite skater was at his meridian form when he won the Olympic gold medal. Meridian can also describe something that takes place at noon, like your meridian boat voyage that launched precisely at noon.