isopleth; isogram; isoline; contour line
Technical term in geography, meteorology, and statistics for a line connecting points of equal value on a map or chart.
Isobars, which are isopleths of atmospheric pressure, are drawn on the weather map.
The contour lines on this map indicate elevation.
等温線 is an isotherm (line of equal temperature), a specific type of 等値線.
Compound of 等 (equal) + 値 (value) + 線 (line). A modern technical term likely coined as a calque of Western scientific concepts such as 'isopleth' or 'isoline'.