isopleth

(mathematics) The straight line which cuts the three scales of a nomograph at values satisfying some equation. Also known as index line.
(meteorology) A line of equal or constant value of a given quantity with respect to either space or time. Also known as isogram. More specifically, a line drawn through points on a graph at which a given quantity has the same numerical value (or occurs with the same frequency) as a function of the two coordinate variables. Also known as isarithm.

A line connecting places of equal value. These values may be physical such as height above sea level, precipitation, and so on, or of human values such as distributions of population, wealth, or transport costs. The word contour may be used as a synonym.