1. One of two or more nuclides having the same total number of protons plus neutrons, but with different distribution, argon-40 with 18 protons and 22 neutrons, potassium-40 with 19 protons and 21 neutrons, calcium-40 with 20 protons and 20 neutrons. The product of a ß-disintegration is an isobar of its parent. 2. The line on a map connecting points of equal barometric pressure. 3. Any curve or equation relating quantities measured at the same pressure.