Isopleth

Curved lines in the plane of the axes of ordinates and absci as, representing the projection of horizontal sections of a curved surface. The absci as and ordinates represent two coexistent quantities (as latitude and longitude, or time of day and time of year, or time of day and altitude above ground) that fix the date or place of any occurrence; numbers are then entered at each point, representing the nature of that occurrence (as time of sunrise, altitude above sea, temperature of the air), and isoplethic lines are drawn connecting points that have equal numbers (as isohyps and isobeths, or contour lines, isobars, isotherms). This term was introduced 1880 by Vogles, but curves of this nature had long been familiar to topographers, and had been used by meteorologists.