This is a line of equal value (a Greek word iso - equal; pleth - value). Examples of different types of isopleths:
Isoneph Cloudiness
Isopycnic Density
Isodrosotherm - Dew point
Isohume Humidity
Isohyet Precipitation
Isobar Pressure
Isallobar - Pressure tendency
Isohel Sunshine
Isotherm Temperature
Isogon - Wind direction
Isoshear - Wind shear
Isotach - Wind speed