1. A line, on a given reference surface, drawn through all points where a given quantity has the same numerical value. The reference surface can be any coordinate plane functionally related to the given quantity (this includes physically defined surfaces in space). This, therefore, is a very general term. Although isopleth is used in this same broad sense by meteorologists, it has a more restricted meaning when used in most other sciences. See also isotimic line. 2. As sometimes restricted, a line drawn through all geographical points that experience the same frequency of some meteorological event.