line connecting points of equal subterranean temperature
• (n.) A line or curved surface passing beneath the earth`s surface through points having the same mean temperature.
I`so·ge'o·therm noun [ Iso- + Greek 
isogeotherm A line or surface (usually imaginary) connecting points in the interior of the earth having the same temperature; an isogeothermal line.
an imaginary line connecting all points within the earth having the same mean temperature. Also called