line connecting points of equal temperature and depth underground
• (n.) A line connecting the points on the surface of the earth where a certain temperature is found at the same depth.
A line or surface showing the depths in oceans or lakes at which points have the same temperature. Isobathytherms are usually drawn to show cross sections of the water-mass. 
I`so·bath'y·therm noun [ Iso- + Greek ... deep + ... heat.] (Physics Geology) A line connecting the points on the surface of the earth where a certain temperature is found at the same depth. 
isobathytherm 1. A line connecting the points on the surface of the earth where a certain temperature is found at the same depth. 2. A line or surface showing the depths in oceans or lakes at which points have the same temperature. Isobathytherms are usually drawn to show cross sections of the water mass. ...
a line on a chart or diagram of a body of water, connecting depths having the same temperature.