A line drawn on a weather map to connect places that have the same temperature. To make an isothermic map, you need to first average all the temperatures in one area (state, national, whatever) during the period of time you want to cover. Once all the average temperatures are laid out on a map, you just draw a line connecting the places that have the same average.

In the Northern Hemisphere, isotherms are very irregular because there are more land masses, all with varying altitudes and temperatures. In the Southern Hemisphere, which has fewer land masses, isotherms are more regular, though they still curve all over the map.