Isopleth

Isopleth
The line or area represented by an isoconcentration.
Source: Terms of the Environment ...

Isopleth
The line of constant condition, (concentration, temperature, pressure etc.).
Isotope ...

isopleth A line of constant or uniform value of a given quantity. See isanostere, isobar, isohaline, isopycnic, and isotherm.
isopycnic An isopleth of density. Constant or uniform in density.

See also: Concentration, Solution, Residue, Infectious agent, Characteristic