The Collaborative International Dictionary of English v.0.48:

Isocheim \I"so*cheim\ ([imac]"s[-o]*k[imac]m), n. [Iso- + Gr. ?,
   ?, winter.] (Phys. Geog.)
   A line connecting places on the earth having the same mean
   winter temperature. Cf. Isothere. Isocheimal