isopleth

A line, on a chart, showing the occurrence of equal quantities, conditions, or frequencies of any phenomenon in its relation to two independent coordinates. This term was first used by Vogler (1880), but the lines themselves had long been familiar.

A line drawn on a map through all points having the same value of some measurable quantity.

thermodynamics A line in a phase diagram indicating the same mole fraction.

a line drawn on a map connecting points having the same numerical value of some variable