geotherm

noun
  1. a line or surface within or on the earth connecting points of equal temperature
  2. the representation of such a line or surface on a map or diagram
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Examples from the Web for geotherm

Historical Examples of geotherm