- A line of equal barometric pressure on a weather map.
19th century. From Greek, meaning equal weight.
A type of contour line you see on weather maps showing areas of differing atmospheric pressure. You can still see these lines used when you watch weather forecasts on television. A basic understanding of meteorology (or at least what kind of weather is coming at you) is useful for sailors because storms and hurricanes are generally to be avoided.
Rogers, J.G. (1985). Origins of Sea Terms. Mystic Seaport.