Isotherm
Isotherm , noun
[Iso- + Greek {not transcribed} heat, from {not transcribed} hot.]
1.
(Physics Geography) A line connecting or marking points on the earth's surface having the same temperature. This may be the temperature for a given time of observation, or the mean temperature for a year or other period. Also, a similar line based on the distribution of temperature in the ocean.
2.
(Physics) A line on a graph connecting points representing states having the same temperature; an isothermal line.