ad·i·a·bat·ic 
(ăd′ē-ə-bătĭk, ā′dī-ə-)
(ăd′ē-ə-bătĭk, ā′dī-ə-)adj.
Of, relating to, or being a reversible thermodynamic process that occurs without gain or loss of heat and without a change in entropy.
[From Greek adiabatos, impassable : a-, not; see A-1 + diabatos, passable (dia-, dia- + batos, passable, from bainein, to go; see gwā- in the Appendix of Indo-European roots).]
ad′i·a·bati·cal·ly adv.