D defs.my
Entry 2 senses Webster, 1913

Isotherm

· I·so·therm · IPA /ˈaɪ.səʊˌθɜː(ɹ)m/
01 n. 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 obs…
  1. 1.
    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.(Phys. Geog.)
  2. 2.
    A line on a graph connecting points representing states having the same temperature; an isothermal line.(Physics)