The Collaborative International Dictionary of English v.0.48:

Isochor \I"so*chor\ ([imac]"s[-o]*k[^o]r), n. [Iso- + Gr. xhw`ra
   space.] (Physics)
   A line upon a thermodynamic diagram so drawn as to represent
   the pressures corresponding to changes of temperature when
   the volume of the gas operated on is constant. --
   I`so*chor"ic, a.
   [Webster 1913 Suppl.]